Crazy Transit Pitches

[luobo]

Thanks for the feedback! Lots to think about.

Personally, I think the biggest weakness of the proposal is that it can't provide the ride directly up Washington Street to DTX -- you'd likely need some sort of surface link at Blackstone to facilitate this (in my ideal world we'd pedestrianize Washington to the Pike, which would help with bus speeds for this specific trip). I suppose the most direct solution to that problem is just redoing the old Orange Line route -- just underground this time -- but branching Orange (especially somewhere central like Tufts) would cause other issues. Surface-level Light Rail to Nubian and RLX-Mattapan are certainly cheaper, but this still leaves a gap between Nubian and Mattapan (I'd hope that RLX-Mattapan happens anyway, turning Mattapan Square into a real transit hub of its own).

As a new person to the forum I'm not totally sure what the Urban Ring BRT you're mentioning would look like, so I can't comment on that.

Some thoughts on potential demand:

I started thinking about this route after looking at the bus networks in Roxbury, especially the Nubian - Roxbury Xing - Ruggles "triangle" where you have a large number of people trying to get to a rapid transit line.

My (admittedly amateurish) read is that this shows a pretty strong demand corridor past Nubian. You've got 19, 23, 28, 44 and 45 (which has a slightly different routing but close enough to count) all coming up Warren/BHA to serve Nubian before bringing to the Orange Line connections. Those five lines together were moving about 30,000 people per day according to the 2018 profiles, and while that ridership doesn't perfectly match on the this proposal it shows enough potential to warrant a closer look. Even if we assume that people are *only* trying to get to the Orange Line, doing so on a train is a lot better than doing it on a bus.

In terms of population, Mattapan and Roxbury have about 90,000 residents at pretty decent density. This is about the same as the population of Quincy when the Red Line was extended out there in 1971 (and Quincy is more spread out). It's more people than Arlington and Lexington had in the 1980's when the Red Line was going to be extended that way (and those towns have much lower population density than this corridor). Considering the number of Blue Line West crayons out there sending it to Brookline or Newton (less dense areas) I'm not sure why Roxbury and Mattapan would have lower demand for an HRT trunk than those places, especially considering more riders there are transit-dependent, which is why tens of thousands of people are willing to put up with slow bus service through crowded streets to get to the Orange Line. I'll concede that these projects were/would be cheaper, but the populations there are also less dense and less transit-dependent so the per-passenger investment might not be as different at the end of the day.

As far as the specifics of the one-seat crosstown, I agree that jogging out to the Esplanade is pretty inconvenient if you're trying to get to South Station or DTX. Having better Regional Rail at BBY would help a lot with the South Station connection, and the OL/GL option would still be there at "Trinity." Connecting with the RL at Charles/MGH is perfectly fine if you're going west, it's really only the eastbound Red Line connection that gets significantly inconvenienced by a few minutes. It's certainly not perfect, but few suggestions are when it comes to digging around in downtown. If we delete Bowdoin (very doable), despite going around-the-world a bit, the actual number of stops between Nubian and Govt Center/State St isn't terrible. Blackstone is the last station on the Washington St alignment, so then you have: Tremont, "Trinity," Esplanade, MGH, Govt Center, State.

I think that it's a little hard to gauge the pent-up demand for the possible trips people would take on this, because it's coloring a bit outside the box. The sentence "Blue Line to Mattapan" would probably strike most people as bizarre until they look at how all the pieces fit. So you don't have many people who have built their potential commutes around a service like this. It's a little bit like the Regional Rail/NSRL discussion: sure, if you look at current demand you'd find very few people aching for a train between Lynn and Brockton. But that's because it's never existed, so transit-dependent people in Brockton would never look for jobs in Lynn and vice versa. If you establish the link, though, the constituency can start to build.

So what I like about this proposal is that it opens up a lot of interesting possibilities:

• One seat from Mattapan to Logan (or Revere/Lynn)
• Very quick two-seat to Kendall
• Opens up a part of the city to transit riders from other lines (how many Cambridge transplants even know that Boston has a zoo?)
• Running Franklin through Fairmount (reducing NEC traffic) becomes more feasible since people going to BBY can still get there via Blue
• (foamer scope creep alert) Keep going south through Milton via 138, past Curry College, all the way to the 128. You could put a Park & Ride garage a la Quincy Adams there that would be positioned to soak up traffic from 95 and 24. With a one-seat ride to BBY, Govt Center, State Street, and Logan (and better price/frequency/span than Route 128), there's potential to pick up southside commuters who aren't going to South Station.

 

[JeffDowntown]

The odds of the NIMBYs letting you run a TBM under that real estate in Back Bay and the South End are zero.

Also, that is the most challenging kind of TBM work (possible, but mega expensive). The subsurface conditions there are soft: glacial till and Boston blue clay, fully saturated with water (to significant depths). The buildings above all rely on that water table for piling stability. Dewater it, and you destroy dozens of historic properties. So the tunneling effort needs to be 100% waterproof. Good luck.

 

[Arlington]

^ I think the turn inland has to be via Charlesgate, where you're not right under the Back Bay's prime real estate.

 

[Riverside]

There are basically two pieces to this idea:

1. HRT from South End to Mattapan via Washington, Warren & Blue Hill
2. Hooking the Blue Line back around from Charles to Back Bay

Part 1 seems more reasonable to me, although obviously you have to figure out how to hook it in to the downtown network. I think a modern non-mixed-traffic-running LRT with level boarding and lengthier trains (more like the LA Metro than SF Muni), hooked into the Green Line network at Boylston, could work. It could be built to allow future conversion to HRT as well.

Part 2 seems a lot trickier, unfortunately.

 

[KCasiglio]

What about an Orange line extension? Branching the OL at Roxbury Crossing, with one line following the existing alignment (eventually to West Roxbury) and the other running under Columbus->Seaver->Blue Hill Ave. Maintains the Back Bay connection without the tricky situation of getting through the Back Bay north-south.

Couple that with an F line to franklin park via Nubian and RLX to Mattapan and we're looking in good shape imo

 

[Riverside]

^ The downside of that (and I'm not trying to be negative here, just honest) is that it halves the frequencies of both Orange Line branches. It's not insurmountable, but it's a definite downside in the quality of service. Look at this way -- whatever frequency you aim for on the branches, you have to figure out how to double that north of Roxbury Crossing. If you want 7-minute headways along Blue Hill, you need to swing 3.5-minute headways in the core, which isn't impossible, but would be challenging. If you ease up in the core to 5 minute headways, you're stuck with 10 minute headways out on the branches, which is starting become unideal.

 

[F-Line to Dudley]

^ The downside of that (and I'm not trying to be negative here, just honest) is that it halves the frequencies of both Orange Line branches. It's not insurmountable, but it's a definite downside in the quality of service. Look at this way -- whatever frequency you aim for on the branches, you have to figure out how to double that north of Roxbury Crossing. If you want 7-minute headways along Blue Hill, you need to swing 3.5-minute headways in the core, which isn't impossible, but would be challenging. If you ease up in the core to 5 minute headways, you're stuck with 10 minute headways out on the branches, which is starting become unideal.

Yes. Halving frequencies before a mega bus terminal is muy bad transit practice. Forest Hills carries so much heft unto itself as a linked-trips terminal that you could never ever have anything less than true mainline-grade frequencies there. The platform already suffers from severe overcrowding at present-day base OL frequencies at peak times when the bus terminal is at its most slammed, and needs both the OLT service densification and some load spreading of transfer routes to Roslindale via a West Roxbury extension to truly get on sure footing for the future. Branching prior to it is ruination comparable to the loss of the Washington St. El for what it kneecaps in terms of linked-trip utility.

This same general principle is why the Mystic Study Group's unicorn proposal for an Everett OL branch is D.O.A. The frequency reductions to Malden Center bus terminal end up a greater downside. Now...branching SOUTH of Forest Hills or NORTH of M.C. would work just fine. And indeed that was once a study flavor down south for twin Dedham and Readville extensions back in the 70's...the Readville ROW now spoken for with NEC traffic needing all 4 track berths for Amtrak growth. Theoretically you could do the same at Malden Ctr. with a short subway fork down Centre or Florence St.'s linking to the Saugus Branch ROW...but then the grade crossing minefield on the Saugus Branch becomes its own inhibitor as the residential out there is extremely unlikely to accept an El looming over their 2nd-floor windows. But best-practices wise that's where HRT branching filets frequencies equitably...after you've hit the mega bus terminals contributing hugest share of linked trips with full mainline frequencies.

 

[F-Line to Dudley]

There are basically two pieces to this idea:

1. HRT from South End to Mattapan via Washington, Warren & Blue Hill

I think for this one you first have to answer the "Why?" question. HRT to Mattapan Sq. is eminently doable. It's even possible to take it further at some later date via a short stationless River St. subway to glom onto the ex- 4-track width portion of Fairmount Line throughout Hyde Park and hit River St., Fairmount, Readville, and points further to Dedham Center (whereas Fairmount ROW in Dorchester all points north of Cummins Hwy. is far too squeezed for dual use).

Blue Hill Ave., however, has never ever been a one-seat ride at any point in its 120-year history as a later-growth trolley suburb. It was a forced transfer at Dudley or Mattapan back when the streetcar was a reservation, and no "universe of alts" proposal since then ever would've given it a one-seat (wrap-around from Mattapan trolley, continuation of Dudley/Nubian GL Branch, anything). It's too long to dispatch that way. So to claim this demands a crosstown one-seat you first have to prove what's changed so dramatically with the corridor that it now demands crosstown one-seats (demographics, big biz, whatever) as opposed to simply re-delivering "equal-or-better" linked-trip utility at Nubian? This won't stand on its two feet as a proposal simply being pitched as a 'more-perfect ____' stitch job smearing benefits diffusely around a map. There's got to be some galvanizing demand--new to the modern era--that's really driving a greater corridor here. As we saw with the Silver Line, the attempted shotgunning of Washington St. with the Transitway did not meet that standard. There was no galvanizing demand for Dudley-Seaport thru, and it was a shotgun marriage of convenience that lacked the corridor bona fides to get itself done. There are too many similarities here with that "why" question.

In the end the Mattapan end is strengthened by Fairmount Line urban rail, and the fact that after this latest 20-year punt on the trolleys they'll be studying their rapid transit options all over again. Nubian we can project gets its semblance of "equal-or-better" back by replacing Silver with higher-capacity Green on Washington with behind fare-transfer linkup to the big downtown transfers reducing the fare steps for making linked trips. And what the streetcar does not backfill at 6 min. headways 100% of the speed and capacity of the old El that's where the Urban Ring Kenmore-Nubian + Nubian-Southie (and JFK spur) radial BRT diversifies the linked-trip options to a depth the neighborhood has never seen before. 28X on the Blue Hill reservation getting lifted in tandem from the diversification.

All of those are officially-studied proposals implementable as single-project cogs at reasonable price, and they address with laser-like focus the neighborhoods traditional (and currently far unmet) demand as a linked-trips center. To change the narrative to this being an HRT corridor you not only have to prove thru-and-thru corridor demand over the mountain of evidence pointing to "directionless" linked-trip node, but also explain where the cheaper study options fall short by not going hard enough at thru-and-thru corridor. That's going to be very difficult to prove with numbers against the whole history or neighborhood transit past/present/projectable-future. Chances are it's going to fall just as flat as the marriage-of-convenience Silver Line at speaking directly to those demographics, because it is largely the same kind of conflicting-interest crayon draw.

2. Hooking the Blue Line back around from Charles to Back Bay

Another "why" when the follow-thru for Green Line's future trajectory sends the E there to eliminate the at-grade Copley Jct. bottleneck and leverages the South End-Boylston hookup instead for sending trains to Huntington. I can't fathom how Orange + Green + all manner of southside RUR overlapping at one megastation isn't going to be enough. As huge as BBY's rapid transit ridership is, it's still smaller than SS, DTX, and Harvard by an order of magnitude. If the E relocation, while further future, is simply too logical on its face not to do then what evidence is there that two rapid transit lines (incl. all the multi-routing utility of the South End and Lechmere ends of Green) ends up not enough and there has to be a third touch? I mean...it's pretty obvious SS needs its thru-service linkup to the Transitway and that such a build would directly keep DTX overload in-check, and it's pretty obvious why the Urban Ring includes the LRT touch to Harvard as a second pipe. What's driving a triple-up at BBY? And why BBY rather than directing resources to another overloaded stop in need of augmentation (like continuing this Storrow jog to Charlesgate & Kenmore).

That demand question is sort of left up to the imagination while it's folded into the marriage of convenience build. It can't be; the Silver Line lesson told us that kind of mission-statement murkiness is too significant an inhibitor for advancing a major project.

Part 1 seems more reasonable to me, although obviously you have to figure out how to hook it in to the downtown network. I think a modern non-mixed-traffic-running LRT with level boarding and lengthier trains (more like the LA Metro than SF Muni), hooked into the Green Line network at Boylston, could work. It could be built to allow future conversion to HRT as well.

Disagree because the evidence is so lacking that this is a thru-to-downtown corridor vs. one that needs a fast/frequent ride up/down Blue Hill to the transfer nodes but then sprays in a zillion diffuse directions rather than overwhelmingly having a missing downtown leg. I'll agree that there's a study inquiry here to better understand Roxbury-Mattapan travel patterns, but given the lack of compelling evidence to-date that the neighborhood's general transit orientation has changed much in the last couple decades the evidence of thru-and-thru demand uncovered in study would have to be absolutely overpowering to advance this. "More convenient for many" isn't going to cut it. You must slice/dice real transit shares and show that Nubian-Downtown is pulling everyone in-tow to Back Bay like a tractor beam instead of distributing them in all directions @ Nubian. And I very much doubt the studies are going to show that so convincingly, because we'd be swarmed with anecdotal evidence for years by now.

Part 2 seems a lot trickier, unfortunately.

Construction-trickier engaging the Back Bay street grid and crossing under a couple tunnels, perhaps. But if you've already gone as far as Dartmouth St. on the Storrow roadpack you've already adopted half of the "Riverbank Subway Redux" conditions of Storrow teardown in exchange for Blue Line transit-trade in same project area with climate resiliency fortified shallow tunneling. So the first question begged is a "Why?" one re: choosing to triple-up Back Bay Station with rapid transit instead of doubling up Kenmore with its tail-track provision for any crayon-drawn direction future extension. The demand question gets skewed by the extreme luxury being heaped onto BBY while Kenmore arguably needs the second future pipe to take a load off for absorbing all manner of Urban Ring service patterns and redevelopment out that way. Plus the corridor traced to Kenmore more fully trades in transit for the deleted Storrow midsection and costs flat-out less with the only cut-and-cover construction being an 1800 ft. diagonal line from Charlesgate to start of Brookline Ave. via Beacon vs. more than twice that under Dartmouth + more station underpins. So you really really really have to have an ironclad mission statement for the BBY triple-up tying the room together as the main attraction. It can't just be "extra". There's too many other mouths to more easily feed than guess on an over-hedge @ BBY.

 

[Riverside]

Part 1 seems more reasonable to me, although obviously you have to figure out how to hook it in to the downtown network. I think a modern non-mixed-traffic-running LRT with level boarding and lengthier trains (more like the LA Metro than SF Muni), hooked into the Green Line network at Boylston, could work. It could be built to allow future conversion to HRT as well.

Disagree because the evidence is so lacking that this is a thru-to-downtown corridor vs. one that needs a fast/frequent ride up/down Blue Hill to the transfer nodes but then sprays in a zillion diffuse directions rather than overwhelmingly having a missing downtown leg. I'll agree that there's a study inquiry here to better understand Roxbury-Mattapan travel patterns, but given the lack of compelling evidence to-date that the neighborhood's general transit orientation has changed much in the last couple decades the evidence of thru-and-thru demand uncovered in study would have to be absolutely overpowering to advance this. "More convenient for many" isn't going to cut it. You must slice/dice real transit shares and show that Nubian-Downtown is pulling everyone in-tow to Back Bay like a tractor beam instead of distributing them in all directions @ Nubian. And I very much doubt the studies are going to show that so convincingly, because we'd be swarmed with anecdotal evidence for years by now.

"More reasonable" ≠ "reasonable enough" -- I just meant that of the two parts to this idea, Part 1 seems less wacky.

But I agree that this doesn't strike me as being top of the priority list. After...

• the rapid-transit-ification of the Fairmount Line,
• the Mattapan Line gets converted to HRT,
• a proper Green Line branch to Nubian,
• 28 layers on at least some level of BRT, and
• BRT links east and west from Nubian are settled in...

once all that happens, then I think this idea is worth revisiting. But not before all of those, and not instead of any of them. And in any case, on a 40-50 year timescale, not 5-10.

So the first question begged is a "Why?" one re: choosing to triple-up Back Bay Station with rapid transit instead of doubling up Kenmore with its tail-track provision for any crayon-drawn direction future extension. The demand question gets skewed by the extreme luxury being heaped onto BBY while Kenmore arguably needs the second future pipe to take a load off for absorbing all manner of Urban Ring service patterns and redevelopment out that way.

This to me is the biggest thing about any Blue Line West idea. God- and funding-willing, Kenmore will someday be a major crosstown hub, with services to Harvard, Allston, Brighton, Brookline, Longwood, and maybe even onward to Nubian. LRT running in the Boylston Street Subway won't be enough to handle all that, so having the Blue Line there -- especially since it will probably make fewer stops on its way into downtown -- will be a huge benefit, all the more so because of the one-seat ride to the airport.

@luobo, I agree with you that most of the options for the Blue Line west of Kenmore are less than ideal. None of the paths between Kenmore and 128 in any direction are amazing fits for the Blue Line (for various reasons -- if you are bored, a lot of them have been hashed out in the earlier pages of this thread and its sister thread, Reasonable Transit Pitches -- if you have specific routings in mind, happy to summarize for you).

However, Kenmore itself is a huge prize, and would be a lynchpin in a whole-scale re-imagination of the Green Line/an expanded LRT network.

 

[KCasiglio]

Yes. Halving frequencies before a mega bus terminal is muy bad transit practice. Forest Hills carries so much heft unto itself as a linked-trips terminal that you could never ever have anything less than true mainline-grade frequencies there. The platform already suffers from severe overcrowding at present-day base OL frequencies at peak times when the bus terminal is at its most slammed, and needs both the OLT service densification and some load spreading of transfer routes to Roslindale via a West Roxbury extension to truly get on sure footing for the future. Branching prior to it is ruination comparable to the loss of the Washington St. El for what it kneecaps in terms of linked-trip utility.

Sure, in a vacuum absolutely but it's worth considering how much of that bus demand is eaten not only by the branch +W Roxbury extension but by inducing transfer demand away from FH and towards Mattapan with it's hypothetical Red/Orange terminal. Existing frequencies should be able to hold at 6mins anyway since the MBTA is aiming to reduce them to 3min as it is. I'm not going to pour over numbers tonight so I don't know if it would be a worthwhile trade off (nevermind whether it makes any sort of sense from an engineering or cost perspective) but dismissing it out of hand based on frequencies that don't need to change at all seems premature.

 

[Stlin]

], I agree with you that most of the options for the Blue Line west of Kenmore are less than ideal. None of the paths between Kenmore and 128 in any direction are amazing fits for the Blue Line (for various reasons -- if you are bored, a lot of them have been hashed out in the earlier pages of this thread and its sister thread, Reasonable Transit Pitches -- if you have specific routings in mind, happy to summarize for you).

If you wouldn't mind saving me the 200 page search, what would be so crazy:

a) about bluing the D branch from Kenmore? There's enough demand and distance there for it to at least make some sense. That's also an old heavy rail corridor, so it should fit. You could also add a CR connection @ Landsdowne. But every stop is a surface stop, so at least rebuilding stations is more... Doable. But yes, I get this would be incredibly expensive on the station reconstruction front. Actually, that entire Rt 9 corridor needs some car traffic relief, and at rush the D actually gets reasonably crowded.

Independent pitch: park and ride infill between Newton Highlands and Eliot, and fork there to Needham.

b) MGH and Esplanade are almost a perfect straight line to Copley and Pru. If you're going to TBM under Back Bay, why not blue the E branch? Not knowing if the Blue lines streetcar heritage makes it compatible with green line tunnels, it probably would be possible to tie into the branch.
You could, with a lot of traffic pain, cut and cover down from Brigham Center to Brookline village and join the D alignment there. Though, you'd lose Back of the Hill (no big loss) and Heath, which would be.

Either way, I think if you're going to add a green line branch down washington to serve Nubian, I would, not being a transit planner, expect the downtown core to need some capacity freed up.

 

[Riverside]

^ Great questions. I'm not an expert, so don't take this as gospel, but here's my understanding...

Blue to Riverside

There is indeed a lot that makes sense about this idea, and 80 years ago it might have been done, in much the same way the current Blue Line and Braintree Branches were old railroads converted to HRT. There are two pretty big challenges, though, that make it less favorable than you might expect.

First, Riverside Yard. This is the Green Line's largest yard and, as nycsubway.org notes, is where most major repair projects happen. Conversion to HRT pretty much cuts that off from the rest of the LRT network, which makes for a Big Problem. You could maybe possibly work out some sort of "time-share" agreement where LRT vehicles are moved at night to the yard, but that's a whole lot of hassle, and still leaves you vulnerable during the day. And there's basically nowhere else to put a new LRT yard, at least not until/unless you build an extension elsewhere out to 128-land. (Like, maybe you could find some place up in Woburn, but, again, it's a Big Problem to solve.)

The other problem, ironically, is the very extension to Needham you propose. Converting Needham Junction-Needham Heights to LRT and connecting to the Green Line at Newton Highlands has been on the books for 80 years, and has been a serious proposal for most of the last twenty years. In fact, if I had to put money on it, I would bet that Green to Needham will be the next rapid transit extension to open after the current Green Line Extension project to Union Square and Medford Hillside finishes. It's still a good ways off, but it's easily the most reasonable rail extension we ever discuss on here.

The problem is that the corridor in Needham is very LRT-friendly, and very much not HRT-friendly. The big problem is a whole shedload of grade crossings in downtown Needham. Tunneling would be expensive overkill, and an elevated would be also. Unlike the current Riverside Branch, the Needham Branch is not grade-separated, which is basically a requirement for HRT these days. (If you go to Chicago, you can find a few HRT grade crossings, but they are very rare, and pretty much not built anymore, and with good reason.)

So if you to Blue to Riverside, it basically closes the door on rapid-transit-ification of the Needham Branch, which then creates huge problems for the commuter rail. (The Needham Line really shouldn't be a commuter rail line and it really creates a headache by mixing in with the Providence Line and Franklin Line, etc.)

So, between those two, Blue to Riverside looks much less appealing.

Branch to Needham

As explained, this is very much a thing -- for a Green Line extension.

Infill for Route 9

This is not so much a thing, in this exact form. A new station between Eliot and Newton Highlands would make for extremely close stop-spacing, and it's in a generally residential village area -- not optimal for a park-and-ride.

As part of the various Needham proposals, however, there usually is a proposal for some degree of park-and-ride at Route 128, between Newton and Needham. This would hopefully soak up a bit of the Route 9 traffic.

Likewise, there are perennial proposals to increase service to Riverside via the commuter rail line from Auburndale. (For example, as a branch of the Indigo Line.) With proper transit-oriented development, this could also pull in some more of the traffic from Metro West (although my understanding is that that parking lot is often filled these days -- well, pre-pandemic, at least).

Blue to Huntington

Well, to be fair, TBM'ing under Back Bay is itself pretty challenging. This is what @JeffDowntown was getting at: Back Bay is built on landfill which makes it seriously complicated to tunnel under. If you aren't familiar with the term -- Back Bay used to literally be a bay of sorts, at the "back" of Boston (which used to end pretty much at the edge of the Public Garden), though "bay" is misleading, as it was more like a marsh. When the Boston & Albany Railroad (today's Worcester Line) was built, it was built along a trestle between what is now Back Bay Station and Allston. This trestle (fun fact) actually was really important in the history of Back Bay because it managed to slow down the water movement from the tides enough to make it feasible to start what we know call "land reclamation" -- basically dumping lots and lots of materials into the marsh to turn it into solid land.

But... the water's still all down there. And all of the buildings in Back Bay were constructed under the assumption that the reclaimed land would stay exactly where it is -- including the water that's mixed in. If you're tunneling down there, and you accidentally drain out some of the water, the reclaimed land will shift, and the buildings will literally crack open. This actually happened twelve years ago when the T was renovating Copley to make it accessible.

So the TBM part is itself probably crazy enough to make someone think twice.

But, let's say that you get the Blue Line to Huntington somehow. (One option that occasionally gets bandied about -- even as far back as 100 years ago -- is tunneling under Boston Common from Government Center, basically aiming for Park Square at Columbus Avenue.)

Totally agree that Huntington is well-worth a subway. The ridership on the E Line is nuts. Check out the figures on page 15 of the 2014 MBTA Blue Book.

Now, as discussed above, running the Blue Line out from Riverway to Brookline Village to Riverside probably is a no-go. However, extending the Huntington Subway from Symphony on out further is definitely something to seriously consider.

In fact, what you describe -- extending the Huntington Subway out to meet with the D Line -- is something we've discussed here a lot and which I think is ultimately one of the best things the T could do to reimagine the Green Line. The D and the E both merit more proper rapid transit than the B and the C -- the D because of its higher speeds and longer distances, and the E because of its ridiculous ridership. It makes sense to join them together. (But, join them together as LRT Green Line, not HRT Blue Line.)

In fact, most of us on here agree that such an extension should go east as well as west -- instead of joining up at Copley, continue east to Back Bay Station, and make your way either along the Pike or in a new subway under Stuart Street, and then join up with the old Tremont Street Subway in order to join the Central Subway at Boylston. That way, you avoid the flat junction at Copley, which often causes delays (and all the moreso if the D and E were joined up), and can leverage the fancy flying junction at Boylston. (But that's all a whole nother story.)

Downtown Capacity

You're totally right that this is a serious consideration. Strictly speaking, an extension to Nubian might be able to skate by thanks to the loop at Park Street and the quadtracking between Park and Boylston. But it would need careful planning.

I'm a little fuzzy on this, but my understanding is that there is also some signaling infrastructure improvements that could be done to improve capacity in the Central Subway.

The way most of our discussions have gone, the largely emerged consensus is that the way to expand capacity of the Green Line isn't by replacing parts of it with HRT, but instead by adding new "legs" so that the whole system doesn't have to funnel through one tunnel. A typical suggestion is adding an "eastern" leg that connects Boylston and South Station. There are differing versions of this idea, but the general concept is that if you create a second place for Green Line branches to go, you can take some out of Park Street and send them to South Station instead, and thus free up space at Park Street for other branches that aren't well-positioned to go to South Station.

There is a thread called Green Line Reconfiguration that goes through a lot of these ideas, but basically there are two main proposals.

1. Tunnel under Essex Street direct to South Station. This was proposed long ago as an extension to Post Office Square, but we usually talk about sending it to South Station and then the Seaport via the Silver Line tunnel.

2. Go south in the existing abandoned Tremont Subway, hook around when you hit the Mass Pike and head east, and then hook north to enter South Station, basically paralleling the commuter rail tracks, and again head on to the Seaport.

I personally favor Option 1, but that is a very long discussion unto itself.

And, just to bring us full circle, the other major relief for downtown capacity is an extension of the Blue Line to Kenmore, where it can provide express service to transferring passengers. Likewise, the North South Rail Link and the basic Blue-Red Connector would also relieve congestion.

 

[Stlin]

^ Great questions. I'm not an expert, so don't take this as gospel, but here's my understanding...

Blue to Riverside


First, Riverside Yard. This is the Green Line's largest yard and, as nycsubway.org notes, is where most major repair projects happen.

First off, thank you for the excellent summary of the issues! I'll go actually read that reconfiguration thread.
Admittedly, I completely forgot that riverside yard existed. Also, was assuming the GLX VMF would supplant many riverside functions; it is about the same size, especially since D absolutely needs a bigger roster than a F Line to Nubian. That said, after reflection, I have to agree with most of your points except for one concern. Wouldn't Blue to Kenmore require literally the *most* tunnel through Back Bay? As in... all of it, all while staying by the Charles?

I do agree that my dream scenario would probably include green @ South through SL to seaport.

 

[KCasiglio]

. Wouldn't Blue to Kenmore require literally the *most* tunnel through Back Bay? As in... all of it, all while staying by the Charles?

More like around than through, since it would fall entirely along storrow and cut into kenmore via charlesgate.

 

[JeffDowntown]

More like around than through, since it would fall entirely along storrow and cut into kenmore via charlesgate.

Yes, the point of Blue to Kenmore is that it is NOT digging under Back Bay. It is running in a shallow trench in the place of part of current Storrow. It does have a tricky crossing under Charlesgate, but the area is mostly park land and highways. When you get to Kenmore, I believe you are back out of the fill zone into original land as well.

 

[F-Line to Dudley]

Yes, the point of Blue to Kenmore is that it is NOT digging under Back Bay. It is running in a shallow trench in the place of part of current Storrow. It does have a tricky crossing under Charlesgate, but the area is mostly park land and highways. When you get to Kenmore, I believe you are back out of the fill zone into original land as well.

And this is 1:1 shackled to Storrow trade-in, because you need certain conditions to hit paydirt for putting it on the board.

1) Something cheaper and shallower than cut-and-cover construction requiring no utility relocations (i.e. treating the 'sandwich' layer of utils in the first 25 ft. of depth between pavement and tunnel roof always being a big cost carrier).

2) Something that has built-in passive flood protection in its base design so that major cost-bloating pumping installations are not required.

3) Something that can simultaneously increase flood protections for the Back Bay street grid.

4) Something that accomplishes the Storrow trade-in goal of moving A) as many daily passengers B) self-contained in the same project area to qualify for optimal funding streams.

5) Avoids known tunnel cost-bloaters under legacy built-up downtown (such as structural underpins of other stations, too-narrow width or angle streets, too much utility spaghetti, too many poorly-mapped ancient streets...e.g. the sins that killed Silver Line Phase III).


#1 is met by the Storrow EB roadpack having no utility layer. You simply frame by re-pouring the Back St. retaining wall as one of the tunnel walls, and then cut straight down on the roadpack at varying depths so that the bare ceiling can be topped off with ~4 ft. of earthen fill for a graded hillside (call it "BU Beach South") gently sloping from the top of the Back St. wall to the pavement level of Storrow WB (your retained 2-lane slow-speed park access driveway). Ceiling depth would thus vary by height of the Back St. wall across the corridor from anywhere from -6 to 8 ft. under the current pavement level to +2 to 3 ft. over. This is compared to -25 to 50 ft. under for cut-and-cover under street. At the Mass Ave. deep cut the roof may even poke as high as 4-5 ft. over current pavement level with the bridge abutments.


#2 is achieved partially as a function of the shallowness and flatness of the tunnel. The mild changes from slightly subsurface to slightly above-surface roof even out the grade changes on Storrow (esp. the Mass Ave. dip) inoculating against the 'storm drain' effect all points from the very shallow under-street Charles platform to the modest change in grade where it slips under the Muddy River. If the existing DCR pump building @ Muddy River were expanded to include the tunnel level at that point of highest vulnerability, the rest of the flat 1.3 mi. flat grade to Charles only needs a passive drainage channel in its trackbed. That's less intensive flood construction than what the full-on existing D portal has periodically had to get in new bells & whistles over the last 50 years because of its pants-shitting scary 'storm drain' potential from the Muddy...so almost divinely cut-rate given the riverbank location.


#3 is achieved by the glomming the tunnel onto the Back St. retaining wall. Because now instead of a semi-porous block wall you've got: A) two sealed-tight concrete walls stopping a waterlogged Esplanade from flooding Beacon St. basements, B) a passive spillway in the form of the water-tight tunnel roof and earthen berm absorbing the rest of any overtopping and directing it back into the basin rather than into basement levels, and C) possibility (optional...probably not needed most places) of raising the actual retaining wall along Back a couple feet higher into a seawall when it's re-poured for the inbound-side tunnel wall.


#4 Kenmore-Charles is the expendable midsection of Storrow that can be offset by free Allston-to-93 travel on a *slightly* under-capacity intracity Pike. If this BLX has stops at "Esplanade" (Mugar Way) serving Beacon Hill/Public Gardens, Mass Ave. ("Beacon") at the 1/CT1 transfer, and "Kenmore Under" it will carry equivalent number of passengers on 6-car HRT trains across +3 stops as the equivalent vehicle diversions from Storrow. It is assumed that the remaining Storrow WB roadpack will be redone as a 2-lane slow park access road and not disappear entirely for accessibility's sake. Speed enforcement is ensured by dragging all 7 of the one-way Back Bay letter streets across Back St. to meet the park road at raised crossings...such that there will be a speed enforcement and friendly pedestrian crossing area every 450 ft. minimum (i.e. short enough intervals that lazy shortcutting and anything >20 MPH won't be possible).

NOTE: Because of the budgetary mechanics of project area -specific trade-ins, you cannot start shoehorning further extensions past Kenmore into this build. Keep those wholly separate and out-of-sight/out-of-mind as 100% independent future efforts. You can daydream all you want about what comes next, but for the funding sources you can feasibly tap on the Storrow trade-in you MUST keep the Storrow trade-in as a wholly independent project whose value proposition is detached from any future crayon drawing. This is a jurisdictional quirk of Charles-Kenmore crossing the streams with the parkway sunsetting and anything past-Kenmore not.


#5 is achieved by #1/#2 comprising four-fifths the build distance and 2 out of the 3 stations, while juicing its bottom line with the neighborhood flood protection enhancements in #3. Crossing diagonal the 500 ft. from the Storrow alignment to to Beacon St. alignment under the Muddy River is not a big production because that's likewise a utility-free jaunt, and the Muddy riverbed itself is shallow (not to mention 100.00% human-crafted in this spot) so the extra tunnel descent is basically what you have to do anyway to transition to 25+ ft. under-street cut-and-cover below Beacon's utility layer. You basically shiv a watertight metal bulkhead under the river from a nearby borehole and then work entirely underneath with no surface disruption. Beacon is then a 50 ft. wide street with 1 single block's worth of utility sandwich to treat. The intersection with Kenmore's Green Line level is at the easterly end clear of all stacked infrastructure like the mezzanine and GL loop levels, and cuts across at a sharp enough angle to not require enormous amounts of underpinning. If there's any temp disruption to the GL level you might be seeing the furthest few feet of platform at the far (Hynes) end temporarily coned off, but that's it. Most of the BLX platform would be offset diagonally from the Green level well underneath the busway's footprint, and then the tail storage tracks would stretch under the start of Brookline Ave. in front of the Buckminster.

Future extension (which again we are NOT lumping in any way/shape/form with this build) is provisioned by going up Brookline Ave. Crossing of the Pike/B&A then allows possible trajectories of: 1) straight up Brookline Ave. to Longwood; 2) bang-a-right under B&A to Landsdowne Station and continue to Allston/West Station/points further TBD; 3) bang-a-right under B&A to Landsdowne Station and eat the D Line (must go this trajectory to stay clear of the messier infrastructure on the C/D tunnel under Beacon); 4) bang-a-left under B&A and bend back to BBY/Downtown or Mass Ave. (can't really see the service utility, but you can Crazy-pitch it). So...basically an acid trip's worth of any-direction crayoning to mount as an encore.



The Kenmore bona fides are massive, as that station will be the changeover point between Urban Ring flavors. If the NW quadrant Ring Cambridge via BU Bridge is built you'll have circuiting LRT patterns from Lechmere, West Station/Harvard Square directs (with large share of diverted 66 riders hopping platforms to the D for Longwood). Plus some alt-patterning from Sullivan and the NE quadrant Ring. Upstairs/downstairs transfers to the SW quadrant BRT half of the Ring to Nubian Square. Possibly some bend-back E's that use Huntington-to-Brookline Village connecting trackage to terminate at Kenmore Loop during peak runs (extra transfer frequencies for the "66'ers" hopping platforms for Longwood). Consider also that if the E gets relocated from Copley Jct. to South End/Back Bay that the Kenmore-Huntington bend-backs can alt-pattern LRT trips to Nubian via Washington St. or the Seaport to help load-balance the capacity-constrained southern-half Urban Ring. You'll have a massively tamed B with the UR-built subway extension to BU Bridge (possibly even with enough bandwidth leftover to start debating a trolley return to Oak Sq.), and you'll have the Needham Branch hooked in as CR trade-in. All in a massive blender. Therefore the ability to pipe 6-car HRT trips a wicked fast 3 stops to Red Line and 4 stops to Gov't Ctr. is a massive load-bearing relief. Most of the Downtown-goers @ Kenmore are immediately going to shift straight over to Blue, while Green re-fills itself as this hybridized diverging-route blender that's a bit more local in orientation...and thus, saves itself a shitload of thru-the-gut of Downtown congestion that can be reapportioned to Ring/radial/branch travel.

And this, in a nutshell, is why I have a lot of trouble lumping excess luxury at Back Bay Station--which is already a par bet to get an enhanced E someday--when ^this^ is the upside uncorked by portioning your build bullets across the spread. Not to mention that if (and only if) the Storrow trade-in gets secured Kenmore is pound-for-pound cheaper to construct than engaging the Back Bay street grid and underpinning 2+ tunnels/stations en route.

 

[F-Line to Dudley]

Sure, in a vacuum absolutely but it's worth considering how much of that bus demand is eaten not only by the branch +W Roxbury extension but by inducing transfer demand away from FH and towards Mattapan with it's hypothetical Red/Orange terminal. Existing frequencies should be able to hold at 6mins anyway since the MBTA is aiming to reduce them to 3min as it is. I'm not going to pour over numbers tonight so I don't know if it would be a worthwhile trade off (nevermind whether it makes any sort of sense from an engineering or cost perspective) but dismissing it out of hand based on frequencies that don't need to change at all seems premature.

Load-spreading to Roslindale is known-known. The extreme level of route duplication down Lower Washington is the source of a lot of the transfer swells that overwhelm the FH Orange platform, so a route map re-draw that culls some termini to Rozzie massively helps. That's been studied enough to be a slam-dunk, and is why the Boston 2030 documentation begs for at least a +1 OL extension to Rozzie if the state is unwilling to consider all the way to W. Rox (though things may have changed now with the Rail Vision putting more urgency on outright conversion).

But, yes, you DO have to posit where the sea change has been in neighborhood demand patterns to mandate a combo terminal at Mattapan. That's the be-all/end-all that makes it a "Transit Pitch" vs. a Civil Engineering Strongman competition in tunneling feet. It's competing in a world of finite resources even at uncorked costs, so Crazy Pitches have to justify themselves at minimum against other Crazy Pitches that do scratch well-defined transit gaps for their money. When an entire neighborhood corridor like Blue Hill Ave. has been transfer-oriented and NOT Downtown-oriented for its entire 120-year history as a densely populated transit 'burb, you have to account for what the unmet need is with their travel patterns now that wasn't there before in any of their documented history of travel. And you have to account for what change IS going to be galvanized by making the build that addresses the ID'd need. It's got to be meatier than "nice to have", or "if you built the one-seat people would get higher-paying downtown jobs".

This being the Crazy Pitch thread of course we we're unusually flexible and/or outright excluding the bottom-line cost. But what's the top-line hook? Fact is that if 120 years of established neighborhood patterns are still holding and have not stealth-changed in some earth-shattering way in the last 11 years since the 28X study, Blue Hill Ave. is a corridor that circuits up to Nubian or down to Mattapan...then sprays itself via transfer in a million different diffuse directions. With no one destination holding down plurality over all others. That's kind of antithetical right there to a reorganization around one-seat trips on the BHA corridor. Now...Mattapan Sq.? One-seat is achieved by the next mandatory 20-year re-eval of the High Speed Line pointing more firmly to the HRT conversion direction. I think we'll be getting there at some point, even if this past year's most recent 20-year punt stayed stet with the trolley shuttle. Nubian? Discussed at-length what was lost with the El and what package of strengthened LRT pipe to Downtown via Washington and strengthened radial pipe with the BRT Urban Ring to Kenmore and Southie combines to hit real "equal-or-better" by enhancing all the areas (higher-capacity to Downtown and higher-capacity radial) that need shoring up. But between Nubian and Mattapan, linked to Downtown instead of spraying in all directions? Either the 28X studies buried some honking huge lede of unmet demand, something truly earth-shattering has changed since that 2009 study, or neither...nothing's changed and it's still a transfer-heavy corridor.

The newest 28X BRT proposal is in direct service to improving the transfer utility of a transfer-heavy corridor. The bucket list of Nubian & Mattapan improvements also direct-target that. A contrary theory that there's a big sea change afoot to one-seat needs to show the evidence that the studies and the current prioritized builds got it wrong. It can't just hand-wave at "well, maybe there's something there." If there were something there calling for radically different thru-and-thru Downtown majority transit orientation it would be exerting some tangible gravitational pull...and us amateur transpo physicists would be getting that "dark matter" gravity signal in our instruments by now and be feverishly deep-diving into numbers changes. You have to ID what/where that pull is, or supposition of maybe future change just isn't enough to stake a place in a hyper-competitive sea of Crazy, Reasonable, and Everything-in-between Transit Pitches. There's no indication of some big overlooked demand signal here.

--------------------

Now...if you want a ____ Pitch that does fit the mold of a transfer-blender corridor...how about radialing the Mattapan-Forest Hills 31 bus with BRT? If Nubian's got the well-studied Urban Ring ultimately strengthening its NW/SE radial bona fides...where's the echo effect to Mattapan? 31's the 15th busiest bus on the system. In olden times the High Speed Line streetcar fleet was based out of Arborway with that route being the main equipment pipe, so the Mattapan cluster ended up being the second-to-last grouping of isolated/non-subway streetcars to be bustituted (last being the Harvard routes). Mattapan cluster lasted until '55. Morton St. is over-wide parkway dreck...the former routing of Route 3 through the city. You could definitely complement the new 28X with a "31X" that uses the southern third of the new Blue Hill Ave. infrastructure then ropes in DCR for BRT featuring on Morton out to Forest Hills. You can then do a reciprocal "30X" job on Cummins Hwy. where instead of clunkily re-striping that over-wide anachronism it gets median-refashioned bus lanes just like the 28X. The 30 was similarly one of the last streetcar holdouts on the system because it has always acted as a radial; it's #54 in systemwide ridership...would be higher if Rozzie had the OL instead of having to dupe Lower Washington to hit the FH overload. See also the 33/24 to Fairmount/Cleary Sq./Readville..."X"-feature River St. up while we're at it.

Having the 28X become the template for its own form of breakaway BRT featuring catered to transfer-blender corridors would be a decidedly right-sized allocation of resources fitting what has long been known about the neighborhood's travel patterns. Rather than trying to "go big" by shoehorning a Downtown megaproject that doesn't neatly fit any of the known demand...why not "go big" by spraying a confetti cannon of radial transfer juvenation all around? Mattapan may not be on the core-mission Urban Ring as we understand it, but it's probably the biggest 'tweener' radial terminal on the system. Chart a course where these clustered "X" builds set the systemwide standard for what a 'tweener diverging point is supposed to look like, and then liberally apply elsewhere.

(Psst...and if you've really got a hankering to graduate to "Crazy" just get the E back to Forest Hills with a home-base Arborway carhourse. Then that 31X radial on remanicured Morton/BHA has streetcar shuttle written all over it.)

 

[Riverside]

Thanks @F-Line to Dudley and @JeffDowntown, you covered the tunneling stuff much better than I could.

EDIT: Forget to add, @Stlin, regarding the GLX VMF, my understanding is that it won't be enough to replace the Riverside Yard, but I could be wrong! It's a good question. I think the problem of the Needham branch still creates significant challenges for Blue to Riverside.

 

[KCasiglio]

There's no indication of some big overlooked demand signal here.

I entirely agree, which is why I said at the outset this whole exercise of HRT to Mattapan via BB was a solution in search of a problem and that improving bus connections would likely be sufficient.

The only thing I took issue with is that branching before FH (or Malden) should be an absolutely non-starter on any proposal.

 

[F-Line to Dudley]

^ Great questions. I'm not an expert, so don't take this as gospel, but here's my understanding...

Blue to Riverside

Don't think Riverside Yard's necessarily a blocker. If the system's going to expand to elsewhere like absorbing the entirety of the north-half Urban Ring, you've got plenty of heavy-repair options elsewhere. Recall that back in the 1945 expansion plan they were going to dual up Riverside with an HRT line (of TBD affinity) on the B&A through Newton meeting the streetcar D at Riverside at a massive dual-mode yard complex. The yard wouldn't have been any bigger than today's. It simply would've been augmented by other sites (Watertown, Arborway, Reservoir...and GLX since that was on the same '45 map) reflecting the more cramped portioning.

For example, there's no shortage of industrial sites for fashioning a heavy-repair carhouse on the Everett/Chelsea side of the Mystic on Urban Ring NE quadrant or on a Massport parcel somewhere outbound of Silver Line Way.

The other problem, ironically, is the very extension to Needham you propose. Converting Needham Junction-Needham Heights to LRT and connecting to the Green Line at Newton Highlands has been on the books for 80 years, and has been a serious proposal for most of the last twenty years. In fact, if I had to put money on it, I would bet that Green to Needham will be the next rapid transit extension to open after the current Green Line Extension project to Union Square and Medford Hillside finishes. It's still a good ways off, but it's easily the most reasonable rail extension we ever discuss on here.

Yes...this one. The 7 grade crossings work spot-on with LRT, but would all be required eliminations with HRT. There's little compelling reason to eliminate: Upper Falls, Gould/TV Place, Needham Heights, and Needham Center stations can all flank their platforms on opposite ends of the crossings with graft-on signals for transit priority and crosswalks. The others--Webster, Rosemary, May, Oak St.'s--simply aren't bad enough for traffic on a pretty well-distributed downtown grid for going all heroic with the grade separation. The only crossing here that *may* be worth eliminating is Gould because of the burgeoning TOD up the street that's likely to sharply increase traffic volumes over the crossing (the actual 128 Pn'R lot here wouldn't factor, being fashioned on the Muzi Ford lots well south of the crossing).

The price difference is stratospheric enough to throw a monkey wrench in an otherwise straightforward conversion from Commuter Rail...which needs to happen sooner because of the NEC SW Corridor traffic pinch's effects on RUR implementation and Amtrak growth. Put it this way: the Rail Vision is cranking this one up fast as a burning priority, so you simply won't have enough time to figure out a way to get Blue to Kenmore before the Needham Branch has already gone Green out of Rail Vision expediency. So the value proposition becomes "Why Blue after it's already gone Green?" and trying to tally up the above-and-beyonds for the later HRT mode change. That'll be hard when Green is plenty improvement by itself.

Infill for Route 9

Yeah...adjust the Eliot stop spacing if this is a thing. But I also agree this really isn't a thing. It's not like there's anywhere along 9 to put a park-and-ride. These are all thoroughly neighborhood stops.

The fact that the D stop selection is so very baked-in from 19th c. re-use of old Boston & Albany branchline stops also means you have trouble with the utilization scale-up to 6-car HRT trains (which, mind you, can get longer with higher seating capacity much closer...if not guaranteed exact...to Orange Line specs once dimension-stunting Bowdoin curve is eliminated) vs. Type 10 supertrains. The only big TOD growers are the Upper Falls/New England Business Center/TV Place trio on the Needham Branch where the capacity "increase" is getting first-time transit at all. All of the D stops are what they are, and Riverside TOD growth will be counterbalanced significantly by park-and-ride volumes diffusing more to Highland Ave. @ TV Place to the south and Fitchburg Line urban rail @ Polaroid/128 to the north. Hence, the coming-attraction parking reductions at Riverside with the full TOD buildout. So you're not really looking at ridership increases to fill a lot of those king-size HRT consists. Arguably, 6 mins. each to Riverside and Needham is going to clear out tons of crowding between Kenmore and Newton Highlands on the heaviest-use portion and fix whatever glitch may exist today. The capacity argument for conversion is weak at best because trains that large are going to run that empty through unchanging village stops.

In short...you could swallow the D to say you swallowed the D. But the relatively tame use of per-train capacity plus all the extra expense for 7 Needham grade separations begs to ask "Why?" Keep in mind, if you get BLX to Kenmore in the first place you are geographically unlimited in where you can go...including Allston/West, a kooky bend-back to Downtown, or alt-spining up Brookline Ave. There's no gun to anyone's head saying they have to have an immediate answer as to where to take it next...and "oh well the D is just sitting there". If you've got a way higher-leverage westbound extension but can't fundraise for it for another 30 years...just sit at Kenmore. The routings will always be there. Make your ultimate choice count to the max.

Blue to Huntington

Put it this way...your highest-leverage touch of Huntington is going to be alt-spining the Central Subway with a dig extension under the reservation then some sort of link-up to the D in the Brookline Village vicinity. Then of course replacing malformed Copley Jct. with a Back Bay stop that ties in via Marginal Rd. (urban renewal nuke zone easier-dig) to the South End tunnel @ Boylston and choose-your-adventure routings therein. Because that's where a radial-distributed Green Line with north-half Urban Ring, Harvard Spur, Porter/Route 16, Seaport, and Nubian can all blend as cogs.

You do have potential HRT crossings...but they're crossings. Like, for example if you took that Kenmore BLX up Brookline Ave. to Longwood and diverted down Fenway + Ruggles or something. But that's different than a straight-on Huntington Subway extension, which very clearly goes in direct service of Green-as-routing-'blender'.

Still would caution against overrating BBY too much. Because if you envision that E relocation as the first step in the eventual alt-circuiting of the Central Subway the Green side of BBY is going to one day be seeing way, way denser than 6-min. headways. With a max-cranking Orange, all manner of RUR'd Purple Line service, and hopefully NSRL taking linking Purple north I can't fathom what would still be lacking at that node meriting an HRT triple-up.

Downtown Capacity
There is a thread called Green Line Reconfiguration that goes through a lot of these ideas, but basically there are two main proposals.

Bingo. Central Subway has a higher native capacity than right now when all the signal/vehicle/dwell cruft gets solved by the ongoing GLT effort. Then it's simply adding better distributing cogs. Take for example how much less-overloaded Park St. would be if there was a one-seat via Boylston to the SS/Seaport, and Red-Blue ended the DTX/Park double-transfer. How many fewer people make the mad upstairs/downstairs scramble? How much quicker do the doors close? 15%? 20%? It might be carrying exactly as many passengers as today from other forms of growth, but it would SEEM like a Green Line station only half as busy because the cloggingest/time-consumingest of ped movements slowing the whole works down would be reshaped far away from there.

Also...since straight-on repeat of the failed Silver Line Phase III routing down Essex St. is wildly impractical and a physically difficult tie-in to the GL level vs. recycling the abandoned Tremont tunnel @ Eliot Norton Park, we probably are looking at a South End jog of some sort. (There are a couple different trajectories so your tastes may vary, but Tremont to the Marginal Rd. traffic island well-studied by the SL III Washington St. tunnel portal looks sanest). So you also have potentially rich multi-routing potential from a 4-track junction at the Pike that includes the E's new Copley Jct.-replacement hook-in. Meaning...if you had E-to-D surface trackage you could right away do a Kenmore-Design Ctr. or Kenmore-Nubian one-seat alt pattern through that junction. You can either/or any of the northern branches--Medford, Union/Porter, Urban Ring NW to Kenmore, Urban Ring NW to Harvard, Urban Ring NE to Chelsea/Logan--to Seaport, Nubian, or Huntington. And that's where the upside of eventually burying the full E to Brookline Village really shows its bona fides. Because then you have multi-directional junctions at Brookline Village, South End, Brickbottom, and BU Bridge to enormously blur services at distributed capacity that dwarfs anything we've ever seen.