Green Line Reconfiguration

JeffDowntown

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Couldn't a green Line tunnel along Essex Street be deep enough to avoid undue impacts on Boylston Station and the Orange Line tunnel? If you started the tunnel to come off the existing Green Line tunnel between Charles Street and Arlington Street, begin dropping immediately and pass under both levels of tracks at Boylston Station (some underpinning of Boylston Station required ) and keep dropping (with a deep bore) to be well under any utilities and foundations along Essex Street, pass under the SB Central Artery tunnel, then rise up to meet the busway tunnel at SS, it seems that it should be doable.

The Post Office Square extension proposed and designed pre WW-1 had a similar trajectory on its west half:

Actual land elevation plays in your favor for this deep bore. Tremont and Washington Streets are the high points in elevation; the land falls off toward Arlington/Charles and South Station. The elevation change is around 50 feet, meaning the route deep under Boylston and Chinatown stations is actually a relatively straight shot from Arlington to South Station.
 

Bananarama

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I love the effort and ideas in these posts.

But hot damn does this just highlight how absolutely stupid the rail layout is in Boston. Just look at all those roughly parallel lines... radiating into downtown as if it's the center of the universe. Way too East-West dependent. Good luck going north or south (let alone across the river) in any reasonable way.

And screw the Silver Line. Stuck in traffic way too much and possibly the bumpiest experience in a vehicle I've ever experienced. I'd love to know the break-down rate of those things having to deal with garbage roads (funnily enough, the last SL1 bus I took had engine start issues at Silver Line Way). -- which actually leads me to a separate question of why does the switch from electric line to diesel require a human attendant? Did the city really skimp on an automatic pole stowing/deploying mechanism? Is that just not physically feasible? Or is this a fleetwide malfunction they're just living with now?

Sorry this is devolving off-topic. This talk about a line along Essex is interesting and more sensible as a layout imo.
 

F-Line to Dudley

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Couldn't a green Line tunnel along Essex Street be deep enough to avoid undue impacts on Boylston Station and the Orange Line tunnel? If you started the tunnel to come off the existing Green Line tunnel between Charles Street and Arlington Street, begin dropping immediately and pass under both levels of tracks at Boylston Station (some underpinning of Boylston Station required ) and keep dropping (with a deep bore) to be well under any utilities and foundations along Essex Street, pass under the SB Central Artery tunnel, then rise up to meet the busway tunnel at SS, it seems that it should be doable.

The Post Office Square extension proposed and designed pre WW-1 had a similar trajectory on its west half:

You're still going to be interacting with the old stations with mezzanines or certainly elevators. How deep are you thinking that would not impact subterranean environs above it?
SL III was hella deep...and got way deeper after the initial scoping turned up a slew of blockers. That didn't end up helping things at all in the end.

Unfortunately we already exhausted this very same angle in an attempt to be less-impactful to the tunnels above, and it was no-go.


There was not ONE fatal blocker. As per last post, the loss of Design funding was a result of there being simply TOO MANY major accrued blowouts on the Essex alignment to make rolling back even a couple of said blockers nearly enough to salvage the project...because too much else wrong would still be hemmorhaging cost overruns to finish 100% design. They were outnumbered/outgunned/surrounded on all sides of the alignment. It was the alignment itself that was the aggregate problem, not their choice of implementation on the alignment.

That's why we're pointing out here that Essex itself is no-go in counterpoint to those stating a 'most-perfect' preference for it, if only we could try harder/do better/muster up more will or something. That's a misread. It wasn't the execution that killed it; it was the properties of the corridor itself. Building it was impossible, because it has racked up the fatal blows way before 100% Design completion by the corridor being the corridor it was. To fix, you must choose a different corridor. The Alternatives range from as few as 2 blocks parallel to as many as 5 blocks parallel, but NONE of them involve any underpins at Boylston or Chinatown, ALL of them engage wider, less densely foundation-abutted streets than Essex, and ALL of them traverse some degree of Urban Renewal zone with better-mapped utility layer. Those specific points check off the self-fix boxes from what combined blowouts got the FTA rec rating and 100% Design funding match repealed. Those are all Corridor-endemic characteristics...not implementation-related. The salvage job simply must go by different streets and engage Boylston + Orange Line from side offsets, not underpins; that's the mission statement for rolling back the Not-Recommended rating and restoring any kind of funding source that can do the rest. We'll do the rest smarter with LRT requiring way less infrastructure duplication at way better performance than BRT...but that was not what upended the original, Essex + the structural underpins were. We could re-mount our BRT fetish on one of these other streets all the same and probably still salvage the FTA rec in spite of it all, because we're going somewhere that's NOT Essex and NOT directly underneath Boylston + Chinatown stations.
 

vanshnookenraggen

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SL III was hella deep...and got way deeper after the initial scoping turned up a slew of blockers. That didn't end up helping things at all in the end.
Do you know how deep the SL tunnel at South Station is? Is it above or below the highway tunnel? If it's below the highway then you could conceivably do a deep bore between Arlington St and South Station. If it's not, you might still have some wiggle room, just way less.
 

JumboBuc

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Do you know how deep the SL tunnel at South Station is? Is it above or below the highway tunnel? If it's below the highway then you could conceivably do a deep bore between Arlington St and South Station. If it's not, you might still have some wiggle room, just way less.
The SL tunnel is above the highway at South Station, and as you travel S down Atlantic the highway tunnel rises up to approach grade around Kneeland St. So the SL tunnel effectively gets pinched between the rising highway tunnel and the street above somewhere in the area of East St, preventing it from continuing down Atlantic.
 

F-Line to Dudley

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The SL tunnel is above the highway at South Station, and as you travel S down Atlantic the highway tunnel rises up to approach grade around Kneeland St. So the SL tunnel effectively gets pinched between the rising highway tunnel and the street above somewhere in the area of East St, preventing it from continuing down Atlantic.
Yes...on the bus loop end of the tunnel on Atlantic it pinches out of spatial room by tunnels above/below for any continuation from that direction. The Phase III hook-in point exists on the corner of Atlantic & Essex as a wall notch, with the junction trajectory basically spanning the middle of the intersection pavement plus the front steps to Two Financial Center (i.e. a pretty wide-angle junction for BRT/LRT). The Atlantic-South block of Essex facing Two Financial and South-Surface Rd. blocks were clean-gutted during the Big Dig for building the CA/T vent stacks and associated underground physical plant and was cleanroomed with SL III in mind. Transition from Big Dig nuke zone to Old Boston street is at the Kingston St. corner opposite the Radian & State St. HQ.

So any trajectory of any kind into SS & Transitway will follow Essex between Atlantic & Chinatown Park next to the Radian...slipping underneath I-93 @ Surface Rd. From the point of clearing the last I-93 wall it's then choose-your-adventure...but this block is the one and only set-aside insertion point into the Transitway because the Loop area further down Atlantic is squeezed off by criscrossing tunnels.


Now...to answer Van's question, the problem with going deeper under Essex west of the Radian is that there is hardly enough incline room to matter after you've thread needle on a totally fixed trajectory between Atlantic & Chinatown Park. Chinatown OL Station with its blowout underpinning costs is only 800 ft. west of there, and that's near the point of max street width pinch. The inclines already had to be of performance-killing variety to slip under the 1908 OL tunnel in only 800 ft. of run-up space, and were already unsuccessful at avoiding building foundation mitigation despite dropping as quickly/steeply as the mode would allow. Mode-switch to LRT and maybe it's not so lethal a performance hit at the same grades...but it'll still be suckily slow from what a sustained descent that must be to clear the OL tunnel, and still scrape the same hull on building foundation mitigation. If you try to steepen the incline some more for LRT's max grades, then maybe you can inoculate more building foundations, but at cost of the performance hit being so absolutely shitty and the constrained upstairs/downstairs transfer to Orange being such a P.I.T.A. that the tunnel won't be able to run representative headways. GL-Chinatown will be a dwell-killer.

At only 500 ft. west of Chinatown, the tunnel still keeps dropping to hit Boylston Under in a deep cavern. On BRT the deep cavern was completely untenable via the performance hit, and because you were now booting every passenger off the buses to loop making the sojurn upstairs to the GL level truly painful. You couldn't go deeper given that ALL from Atlantic was on a thread-the-needle trajectory set by criscrossing Big Dig tunnels, Orange Line tunnel, and max descent between them. Boylston was simply too close to drop down any more...and dropping down as much as they could possibly drop down already failed to ameliorate the blowout underpinning costs of the GL level + Common archeology. They already tried to go most of the way to Arlington with the Charles St. South portal approach underpinning GL, but while that proved feasible the Design was already fucked by Chinatown + Boylston where the depth could not be changed.

So, no...simply going deeper isn't an option from where any Essex trajectory must start. It's not enough running room. As before, Essex corridor ITSELF is the point of failure. It's no-go anywhere up that street because of the way insertion angles are set in stone by the thread-around other tunnels.
 

vanshnookenraggen

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Screenshot 2021-01-13 190427.jpg


Ok so let's go down this rabbit hole. Instead of Essex, use Avery Pl. That curve buys you an extra 200' (give or take) which maybe buys you enough extra grade to where the performance isn't compromised? Or perhaps it doesn't because now you have to go even deeper to avoid building foundations.
 

vanshnookenraggen

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*hits blunt*

OK, follow me here. F Line brings up the good point that other crosstown streets, like Kneeland, are much better for a subway extension. This would then bring up the possibility of the Stuart St Subway. Now my idea in the past was to do it like originally proposed where it was a connection to Huntington Ave. But what if it was, instead, a Boylston Bypass? Reroute the B/C (and currently D) at Hynes to a new tunnel under the Mass Pike (really the only way to get past the Pru). Then it runs along Stuart to Tremont. At Tremont it splits with two services going up to Park St (or where ever) and one going down Kneeland to South Station. The only section of track that would be taken out of service (not abandoned to keep some redundancy) would be between Hynes and Copley. This is more involved than my previous plan but perhaps it is easier to build.

I'm sure some would argue for an alternative route hugging the Pike but I think that's too far away from the CBD of the Back Bay. The South Boston Line (whatever letter you want to use) also wouldn't need to have a transfer to the OL since, if you need to go that way, just take a different GL branch.
 

Riverside

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I picked a helluva time to toss a 2100-word conversational grenade and then peace out for days to deal with IRL stuff, lol. Let me see if I can wade back through the points that have been raised.

Boylston Bypass via Stuart and Kneeland
So, gonna start with this one because this is actually where I was hoping the conversation would go. @F-Line to Dudley, I basically don't disagree with any of the ideas you've laid out in principle -- I think most of them are fundamentally sound. But my core objection is that there's not enough throughput capacity downtown long-term to handle an expanded Green Line precisely due to the combination of the lack-of-Commonwealth-access-to-Kenmore-Loop and the very-close-to-Downtown-branching-that-Nubian-service-would-require. And I'll respond to some of your specific points in a moment, but I'm here to tell you that I've run the numbers and it's still a problem, even with virtually all of the supplements you've proposed.

Do I think any of that should preclude building a Bay Village loop in particular? No. I'm pretty much convinced that it is the best way to connect the old Tremont tunnels to both a branch to Nubian and one to Huntington. And the addition of a north-originating short-turn loop will be extremely valuable. I do think it should make us think twice about a Bay Village-to-South Station connection, and I do think we should consider ways to run through from Blandford St to literally anywhere that isn't Park St.

And that's where Van's (second) idea comes in. This alignment solves that problem by giving Kenmore trains a second destination downtown; those Seaport-destined trains can then be replaced by Nubian originating trains into Park St.

The problem is that a train coming east on Boylston Street literally has only one place it can go: north to Park St. That's gonna hamper any sort of fileting Reconfig concepts. If we can come up with an elegant solution, it balances out the whole system.

(Anecdotally -- I did a similar TPH exercise the other day for the north side, assuming a Brickbottom junction that connects Sullivan to Lechmere, Grand Junction and Porter, and that connects Lechmere to Grand Junction and Porter. The difference is night and day. Having the full junction -- rather than Bay Village+Boylston's half junction -- makes the Reconfig concept absolutely sing.)

Now, I actually toyed with something similar to Van's idea, but perhaps a little more workable. Instead of branching at Hynes, one possibility would be to branch at Copley, reuse part of the existing tunnel and hook that into a new subway along the Mass Pike. (You would need to add a new leg to the junction at Copley, which is potentially non-trivial, looking at the footprints.) Then, like in Van's idea, you could send some trains to Boylston and some to Seaport (and you can even keep your Park-Seaport hookaround.) I excluded this from the write-up mainly because, damn, it was already long enough.

In order for this to accrue any benefits, you would probably need to find a way to eliminate the flat junction at Copley in practice. I guess you could do this by simply running a short-turn dingy between Park and Copley. It would be the only service using the inner tracks at Boylston, but assuming you build a crossover east of Copley, you could do it, though it would be a pretty poor service. (Maybe the flat junction could be less of a problem since there would be fewer trains along that stretch anyway? I'm not sure.) Likewise, you'd need to do something elegant with the new junction at Bay Bay... otherwise you're just trading one problem for another.

Map (and boy did I do a terrible job at Bay Village itself -- just assume that the connections are pretty much exactly as we've been discussing this whole time):

South Side Alt 5.png


@vanshnookenraggen -- was your vision that the Huntington Subway would run into Copley? Or continue to join up with the Boylston Bypass?

And yes -- I argue for a hug-the-pike routing. The only place either alignment would actually have an intermediate station is at Arlington Street; for a Stuart Street subway, that would be absurdly close to the existing Arlington station, and so I think hard to justify. So if you're gonna have no intermediate station west of Back Bay, then what does it matter whether via Stuart or the Pike?

Nit: When the Blue (or Green) goes along Storrow, I think it only needs 2 double-ended stations: MassAve-Gloucester and Clarendon-Berkeley
Yeah, I'm pretty agnostic about placement of stations along Storrow. Maybe I should've marked them more tentatively on the maps. I think I could be convinced of anything between 3 and 0 intermediate stops between Kenmore and Charles.

Shockingly, I'm hitting the character limit again, so I guess it's time for a double post.
 

George_Apley

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*hits blunt*

OK, follow me here. F Line brings up the good point that other crosstown streets, like Kneeland, are much better for a subway extension. This would then bring up the possibility of the Stuart St Subway. Now my idea in the past was to do it like originally proposed where it was a connection to Huntington Ave. But what if it was, instead, a Boylston Bypass? Reroute the B/C (and currently D) at Hynes to a new tunnel under the Mass Pike (really the only way to get past the Pru). Then it runs along Stuart to Tremont. At Tremont it splits with two services going up to Park St (or where ever) and one going down Kneeland to South Station. The only section of track that would be taken out of service (not abandoned to keep some redundancy) would be between Hynes and Copley. This is more involved than my previous plan but perhaps it is easier to build.

I'm sure some would argue for an alternative route hugging the Pike but I think that's too far away from the CBD of the Back Bay. The South Boston Line (whatever letter you want to use) also wouldn't need to have a transfer to the OL since, if you need to go that way, just take a different GL branch.
It's a cool idea. So what happens to Arlington and Copley? Are you keeping E on that route at fairly low headways? It does make some odd crossings without junctions or connections – Orange is lacking as you mentioned but also the Copley Junction crossing over the new bypass is counterintuitive. Why not just keep running E into the Stuart subway from Huntington? If we can swing D-E connector at Brookline Village we can get Riverside/Needham trains going towards Seaport as well.
 

George_Apley

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For what it's worth, here's a reminder for non-transit mapmaker obsessives of what the Bay Village interchange entails
GLX-Bay Village Junction.png


The Bay Village junction (at a full buildout as shown here) allows for Seaport trains from points north via Tremont and west via Huntington. If Huntington is connected to the Riverside line at Brookline Village, trains from Riverside and Needham can access Seaport as well as passengers from points west of Kenmore if some Seaport trains can be scheduled to turn inbound at Brookline Village and short-turn at Kenmore (I'm not sure if D trains can currently short-turn inbound-outbound at Kenmore).
 

Riverside

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Part II of Charlie-knows-how-many...

For me the biggest challenge with the Essex Routing isn't the Essex St tunnel itself (though that does have challenges), it's the tie-in to Green.
Yeah, I tried to summarize this in my original post, but may have cut it for space. The biggest problem with the Essex St Subway is Boylston Station. Whatever mysteries await us under Essex are irrelevant until and unless a solution at Boylston can be found. I reread the entire thread as I was thinking through all this, and I think one of the best summaries I found was that "an Essex Street Subway would need to dive deep under Boylston and Chinatown alike before rocketing back up to hook into the Transitway." It's absolutely for sure a serious problem.

My advocacy for an Essex subway isn't in support of an Essex subway per se, but rather in support for somewhere to divert Commonwealth trolleys to (i.e. the ones which cannot access the Kenmore loop). Essex makes it easiest to illustrate the benefits of that kind of somewhere, but I think it's really just an example.

One comment:

Essex St routing would probably require a lot of work to be done on the Boylston St subway west of Boylston station. In order for the Green Line to be able to hook up to an Essex Street subway, we have to route some Boylston St subway trains off of the main-track that goes to Park and sending them down to a Boylston Under station that underpins the old turn-of-the-century station and then continues under Essex to meet the Transitway.
Essex St subway wouldn't be accessible from the Tremont St subway at all (unless you really blow everything up for a whole new set of flying junctions). It would also probably require getting rid of the defunct but extant flyover junction that continues south on Tremont, effectively blocking future extensions to Nubian.
Sooooo one thing to throw out there is that I'm not convinced that a Reconfigured Green Line actually strictly speaking needs a one-seat ride between Arlington and Park Street. Assuming a Blue-to-Kenmore connection, a Kenmore-originating rider could connect to Orange at Chinatown (or Tufts, if we take some of what Van is smoking) and Red at South Station (plus change for Park at Boylston). With a decoupled north-south subway upstairs and an east-west subway downstairs, both subways would be running at their full capacity, which basically ensures you're waiting less than 2 minutes for a connecting train.

Eliminating the dog-leg eliminates reverse-branching, and also incidentally would easily segregate the streetcar-esque B & C from the interurban-esque D & Needham, and would provide much higher frequencies to the Seaport than could ever be provided from the Park St faucet. (I also am pretty viscerally opposed to trying to pair-match B & C with any of the possible northside branches -- D & Needham are much better fits.)

But to tie back to the Essex Subway -- if you eliminate that dog-leg as a requirement, it would make it easier to restructure Boylston into a simpler two-level "+ shape" station. You can raise the westernmost Tremont track up to level (since it no longer needs to fly under the Boylston Street leg), and dig a shallower tunnel for the east-west subway. You still have to contend with Chinatown, but in theory you could leave most of the existing Boylston St station intact (especially since the station itself is almost entirely north of Boylston Street).

To be clear -- it's a moderately massive proposal. And absent a proper non-BRT study, an Essex subway itself is not per se the one I would most favor. But I think eliminating that dog-leg and creating an east-west tunnel (whether via Avery, Essex, Kneeland or Hynes-Back Bay-Bay Village) opens up a world of possibilities. Chief among them -- in one fell swoop, you double the number of LRT trains you can send into downtown. For all its benefits, the Bay Village loop on its own still sends every single LRT train into either Park or Government Center. Whether it's 40 tph or 60 tph, a new east-west subway literally gives you another 40 tph to work with. It's basically a whole new subway line through downtown.

For ~3000 feet of tunnel? I'm just saying, it does a lot -- and not just for access to the Seaport.

(I'm not sure if D trains can currently short-turn inbound-outbound at Kenmore).
Yes, they can. D branches off of the C tunnel about 1700 feet west of Kenmore, which means they enter on the inner tracks, which means they can short-turn at Kenmore. And thanks for adding the map -- always a useful visual, and better than mine.

Honestly, I'm gonna practice some bad forum etiquette here and reply to F-Line separately, in the name of readability.
 

George_Apley

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My advocacy for an Essex subway isn't in support of an Essex subway per se, but rather in support for somewhere to divert Commonwealth trolleys to (i.e. the ones which cannot access the Kenmore loop). Essex makes it easiest to illustrate the benefits of that kind of somewhere, but I think it's really just an example.
If the issue is Commonwealth Ave trains why not just divert some BC trains to D via Chestnut Hill Ave and Reservoir? Gets them off alignment (IF E-to-D connector).
 

JumboBuc

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Sooooo one thing to throw out there is that I'm not convinced that a Reconfigured Green Line actually strictly speaking needs a one-seat ride between Arlington and Park Street. Assuming a Blue-to-Kenmore connection, a Kenmore-originating rider could connect to Orange at Chinatown (or Tufts, if we take some of what Van is smoking) and Red at South Station (plus change for Park at Boylston). With a decoupled north-south subway upstairs and an east-west subway downstairs, both subways would be running at their full capacity, which basically ensures you're waiting less than 2 minutes for a connecting train.

Eliminating the dog-leg eliminates reverse-branching, and also incidentally would easily segregate the streetcar-esque B & C from the interurban-esque D & Needham, and would provide much higher frequencies to the Seaport than could ever be provided from the Park St faucet. (I also am pretty viscerally opposed to trying to pair-match B & C with any of the possible northside branches -- D & Needham are much better fits.)
I may be totally missing something (and apologies if I am) but wouldn't this basically forfeit service to Copley and Arlington? How does a Cambridge/Somerville RL rider or Eastie/Revere/Lynn BL rider now get to the commercial high spine core at Copley?
 

Riverside

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[Sorry, got pulled away.]

F-Line -- I've been going backward through the backlog of posts, and I see now that some of my responses don't address your points about the Essex subway -- that's my bad, it wasn't my intention to ignore you. My point with bringing up the Essex subway isn't to advocate for that alignment per se, but to call attention to the idea of a new east-west outlet for Kenmore-originating trains, especially from Comm Ave where they can't be supplemented with short-turns. I'll happily concede that the Essex alignment is a firm no-go, but I'd raise that an alternative should be considered.

Re "I'm not sure I agree on the 'Park St. can't support it' basis":

First off, ironically, this entire concept was kicked off by the exchange you and I had back in December about Central Subway capacity. Rereading it now, I do see where some of the misunderstanding arose, but I do have to laugh.

I'm not indifferent to the possibility that the 40tph ceiling could be raised, but I still think it gets tight faster than you would expect. Like I said, I've spent a looooooooong time experimenting with these numbers in a spreadsheet, but I'll try to go through a brief illustrative example.

Comm Ave pre-covid gets a train every 6 minutes during rush hour, for 10 tph. 92% of ridership comes from stops east of Chestnut Hill Ave. (@George_Apley this is why Chestnut Hill Ave helps a little, but not much.) 72% of ridership comes from BU West through Chiswick Road -- i.e. the stretch that would be uniquely served if a branch to Harvard and/or Grand Junction were layered in east of the BU Bridge. For the sake of argument, let's say you reduce those 10 tph to 8 tph, under the theory that it only needs 80% of the capacity if you have that supplemental service east of BU Bridge. I don't think you can justify dropping the service any lower than that. And there is nowhere else to feed that line, other than Park St.

Now let's talk about that branch between Harvard and Kenmore, which also can only be fed by Park St. The 66 pre-covid was scheduled for 9-min AM peak headways. Given the hopes for development in that area, plus the critical nodes this service would uniquely connect (Harvard and Kenmore), I think you at least need to match that. We'll call that another 7 tph.

Now let's review Nubian. I previously laid out my reasoning to argue that this branch needs 10 tph directly to Park. Maybe we could swing 9.

Just with these three, we are already eating up 8 + 7 + 9 = 24 trains into Park Street.

The peak headway standard on the T right now is 6 minutes or better. The exception is the Red Line branches, where it drops to about 8.5 minutes, or 7 tph. So let's use 7 tph as our average frequency for any other additional branches. (I know, I know, not all of the branches need to run to Park Street... I can go through them one by one, but enough of them would still need to that it's going to be a problem either way.)

Most of our proposals range from somewhere between 3 and 7 additional branches. At the extreme low end of that (wherein E and D are combined, and Riverside and Needham each get half frequencies, you keep Beacon through-running, and you add a branch to the Seaport), you'll already breaking the ceiling at 45 tph. If we go middle of the road and say 5 additional branches (for Harvard, Grand Junction, Comm Ave, Beacon, Riverside via Huntington, Needham via Fenway, Nubian, Seaport), and you're already at 56 tph -- i.e. almost half again what the station currently does.

(And, not for nothing, but with hopefully longer trains than what we currently have now.)

Maybe I'm just not fully appreciating how majorly the reduced dwell times can increase throughput. But in the scenario I just laid out, the majority of Comm Ave riders (comprising nearly a quarter of existing Green Line surface ridership) still see degraded frequencies, and the Park St-South Station jog that is intended to reduce transfers gets served by -- at most -- one northside branch's worth of trains.

Am I saying that we need to plan to send trains to all 8 of those destinations? No, of course not. What I'm saying is that the bare minimum of what we talk about here with a Reconfig'd Green Line already requires that ceiling to be raised, and any modest expansion of the network -- absent some sort of Kenmore-South Station connection -- would require capacity increases of 30% or more. This is what I mean when I say this becomes a real challenge very quickly, regardless of whatever the future holds, and regardless of whatever magic we can work by diverting trains out of the Central Subway.

I'm not saying it's impossible that "fewer transfers at Park" and "more Blue transfers at Kenmore" will do the trick, but I'm skeptical.

I'm not seeing where there's a looming crisis in all that causing ANY hesitation whatsoever in building the flexi junctions.
Ah, this is a good point: I 100% do not think that any of what I've laid out should cause any hesitation or delay in building the flexi junctions. (Nice term -- beats multi direction junction.) With the exception of the connection between Bay Village and South Station -- which is not integral to that build -- I think every single flexi junction we discuss here should barrel ahead full-speed. My point isn't that they are wrong to build -- my point is that I think they're insufficient.

And even if there were, absolutely no one is putting a gun to dispatch's heads to saturate each and every flexi junction with service levels to their outermost capacity limits. If on further review the comfiest ceiling is a little lower than breakneck-max...so be it. Maybe your South Huntington service to Hyde Sq. or Forest Hills rides the Copley Jct. alt route to Park Loop exactly like the old days while trunk Huntington service takes the South End thru-and-thru...neither interacting with each other on any track past the new level split abutting Prudential. For the ROI you're still exponentially expanding service levels and service routing flex as a Very Good Thing™, so glass-half-empty concessions like ^^getting extra-creative with track sharing keep-away^^ are part/parcel an outflow of the extra flex. We aren't hurling ourselves headlong into new-creation walls by embarking on this; we're actively avoiding such bottlenecks, even when actual traffic modeling games out on the more-pessimistic side than our best crayon-draw hopes.
This is fair, but, again, even in a modest 7-branch build (Harvard, Comm Ave, Beacon, Riverside via Huntington, Needham via Kenmore, Nubian, Seaport), each of those corridors pretty vitally needs thru-running to Park St/GC -- in some cases because you need a connection to Blue and in some cases because you can only run those trains to Park (and ironically, those groups are nearly completely distinct). This isn't "all trains to everywhere", this is "basic fundamental service" in almost all cases. And, as I've laid out, the art of the short-turn only helps us in the places where we don't need it -- Beacon (low ridership), Brookline Village (short stretch), Huntington (already doubled if not tripled with D, E and Needham) -- but physically cannot help us where we do need it -- Harvard, Comm Ave (lack of Kenmore loop), Nubian (too close to downtown).

Is there a perfect solution? Probably not, and I'm definitely happy to focus on making sure that the perfect doesn't become the enemy of the good here. But I don't think this is a problem of obsessive completism.

Continued below.
 

Riverside

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A couple of additional comments:

Take for instance the Ring largely displacing the 66 bus's crosstown audience, achieved primarily by having Kenmore run-thrus from the Harvard Branch. Since there won't be corresponding SW quadrant LRT owing to lack of ROW's and the upstairs bus station transfer to Longwood/Nubian BRT will be somewhat capacity-limited...you probably achieve ideal load balance by wrapping some E supplementals inbound @ Brookline Village to loop at Kenmore Loop. Steps up the frequencies for the 66'ers to have more Longwood trains, while the extra trains stay 100% track-separated on the C/D-to-C/D loop from all the B/Ring service heading thru on the other sides of the island platforms. The terraforming of the 66 trunk via LRT is thus a service that doesn't hit the Central Subway equally with all of its routing components given how the Loop is leveraged for the quick cross-platform Longwood transfer.
Now, this is a good idea. Huntington would benefit from higher frequencies than the rest of the combined D + E west of Brookline Village, so using Kenmore as a short-turn location would actually work very well. You could also send this route directly out to the Seaport, which helps eliminate a branch that otherwise has to feed into Park.

Take as another instance the art of reaching Nubian direct from Kenmore. Streamlined enough the BRT you'll have to live with on the south-half Urban Ring should be plenty good enough to bear its representative ridership load, but in similar fashion to the "66'er"/Longwood augments there's nothing stopping you from running wraparound E's from Kenmore Loop to Brookline Village...down Huntington...then run thru the wye at Bay Village flexi-junction straight on to the Seaport-v.-Washington split instead of continuing into the Central Subway like most E's. Now...this probably doesn't need to be a frequent filet pattern to do its stated job of keeping the BRT SW Ring buses appropriately free from crowding and over-long dwells. Maybe one turn every 12 mins. does the job. But it can do the job without touching any trunkline track, which in turn takes small but meaningful edge off Park St. TPH management in addition to directly helping the more-direct routed BRT Ring's OTP immensely. When the alt routes such as these are small-stakes but evenly spread around the network, the traffic savings start to add up on the trunks of greatest concern.
I used to think that Kenmore-Brookline Village-Huntington-Bay Village-Nubian was a quirky but intriguing idea. Then I mapped it out, and realized that I actually think it is bananas. No one would ever ride it. The route takes over 6 miles to traverse a 1.5-mile distance. My guess, based on current travel times (and assuming a Huntington subway) is that the journey would take close to a half hour -- it would be only a bit slower to walk. At most, you would see riders using it to travel to Bay Village (or less so Brookline Village) and then transfer; maybe some riders would travel from Nubian to Back Bay or Symphony, but once you hit MFA, a properly enforced bus lane would almost certainly be faster and more predictable (since, I agree, it would only be one train every 12 minutes-- no one's gonna wait for that.) Yes, this would be a way to get a couple of extra trains down to Nubian, but practically speaking it would just be a Nubian-Bay Village short-turn as far as those riders are concerned. Better to use Kenmore-originating trains to divert Huntington service out of Park and free those slots up for extra Nubian trains -- then at least you'd get some Huntington riders riding end to end and more Nubian riders going straight to Park, instead of the train emptying out at Bay Village.

Similarly any 'circuit' Ring service going Lechmere and bending back in at BU Bridge to Kenmore can potentially use Brattle Loop on one end and Park St. Loop on the other end to stay completely traffic-separated from all else on the Park platforms-to-GC platforms stretch of maximal traffic pinch while still covering every single station and transfer on the circuit. That all helps a ton, as it will not need to tally on the "40 TPH" thru-Park tracks at all with that bit of artful-yet-completist avoidance.
Either the inner tracks at Park help contribute to 40+ TPH or they don't. We've already established that adding a crossover to the fence track is necessary to support increased throughput, which presumably means the fence track would be used for through traffic, which means the Ring service wouldn't be separated from the rest of the traffic. Yes, I agree that Park short-turns and Brattle short-turns will become vital in years to come, but it doesn't help this particular issue. [EDIT: I should say -- if the ultimate release valve is to say that, if all else fails, all but one of the Kenmore routes will short turn at Park, and will keep the inner tracks isolated to those services, I could get on-board with that and be persuaded that you could get enough throughput to support these substantial increases in TPH. I'm also a fan of this idea in general because I think the legacy streetcar branches are better off if isolated from the rest of the system, as much as possible.]

Similarly, you can template the old "Arborway-Park"-vs.-"Heath-Lechmere" service overlap differentiators on some of the more expansive linear pokes. For example, if Heath begats Hyde Sq. begats potentially Forest Hills you're probably not running all that thru to the Union/Porter branch. In that case assignments work better with GC looping the outermost streetcars, Union/Porter taking up something differently load-bearing, and the routes with the most streetcar miles having sparser headways than the branch & Ring trunks with greater grade separation. But can also work with 12 min. Forest Hills-GC headways being paired with 12 min. Forest Hills-Kenmore Loop headways via Brookline Village to flesh it out while playing keep-away.
Forest Hills-Kenmore Loop + Forest Hills-GC is a solid idea. I'm skeptical that Arborway restoration will ever happen, but I'm more optimistic about a Hyde Square (and maybe in a 1000 years, Jackson Square) extension, and the same thing would work well there.

Continued below again.
 
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Riverside

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Sort of the same way Nubian can get a targeted augmenting via Kenmore + Bay Vill for backstopping the BRT south-half ring and also boosting Washington St. streetcar headways without a corresponding load on the Central Subway. I could see similar--if more complex--dynamics working for Oak Sq.: (1) BC vs. Oak frequencies coming out of St. Paul portal get a differing 'tweener' slice of the pie for slightly denser-than-6 min. headways to Packards but slightly sparser-than-6 headways after they split; (2) service weights a little more to the Brighton side of Packards split than Comm Ave.; (3) C trains are permanently extended via Chestnut Hill Ave. to BC as an outer service-increase augment (counterbalanced by -3 stop consolidation on Beacon as prereq), leaving only the most ridership-diffuse area of B up the hill between Harvard & Chestnut Hill Aves. dealing with the sparser headways; (4) the respective Comm vs. Brighton Harvard Ave. stops each get countdown timers for the other branch's service across the block for last-minute changes.
These are all good ideas, but they still fall prey to the Park Street faucet problem. Sounds like you're suggesting ~16 tph combined on Comm Ave to Packards, plus perhaps another 7 tph on Beacon, all of which need to thru run to Park. Even if you short-turn half of those Beacons, you're still already at 20 tph, and you still need to feed Nubian (that's 29tph), Seaport (36), some sort of Urban Ring service to Harvard and/or Grand Junction (43), plus some sort of nominal service from Huntington to Park (50), and I guess Riverside and Needham each get 15-min headways to Park + GC at this point? Again, this all adds up quickly. If you want 6-minute headways on anything that runs into Comm Ave, you have to pay for it elsewhere.

In large part we AREN'T hyper-focusing on the flexi-junctions as sole center of the New World Order. Blast-from-the-past via creative use of the loops to cut loads without touching the most-congested trunks is a big part of the service increase. So is asynchronous loading (e.g. Oak > BC @ Packards if the C shores up the other end; multiple pathways to Nubian by tandem BRT & alt-LRT Ring'ing; South Huntington being an open multi-routing prospect) where that shoe fits the overall traffic profile, esp. when streetcar mileage has to be dispatched. Throw enough of that confetti at the board and you do accumulate offsets for the Park area of max traffic concern.

And finally...just note how the gravitational pull overall from the radials starts and ^^all of the above^^ creative service fileting and short-turning starts slowly but surely moving away from Park St. as singular center of the universe. GC becomes way more important with the combo of Red-Blue and Brattle Loop taking up load-bearing properties and prying double-transerees away from milling around the Park platforms keeping doors open. And if BLX-Kenmore ever comes to pass, then the torch is thoroughly passed to GC >> Park as a transfer of magnitude and the bulk of your dwell-driven loading problems @ Park truly perma-disappear. It'll still carry mega thru ridership, but more people will be staying put on the trains when stopped there rather than running like ants all over the platform and stairs solely for the transfer. And that...depending on what magnitude of shift you shoot for in the gravitational pry-away...is worth its weight in substantial TPH increases by virtue of doors closing and trains starting in half the time.

It's hard to parse because all of these benefits are buried in there as part of the 'exponentialness' of the service increasers. You just have to slot your build priorities by bang-for-buck orders of magnitude to reap the benefits. Whether those benefits are exactly as broad as your best hopes would be or somewhat less due to unforseen constraints...it still substantially moves the needle of traffic management because of its innate 'exponentialness' at reshaping. That's why Reimagining is way more a new base philosophy for LRT in Boston than any discrete series of builds.
Look, at the end of the day, I agree with you in practical terms. I'm in favor of all of the flexi junction builds, and likewise the GLT projects (including at Park St), and I'll be dancing the jig if I ever ride a trolley one-seat from Kenmore to Nubian. And I agree that Reimagining offers an extremely fruitful and politically effective strategy for approaching LRT in Boston in the decades ahead.

The core of my objection: the current limit at Park Street is understood to be 40 tph. An extension to Needham can be offset by a combination of D-to-E and slightly reduced headways to Riverside, and an extension to Nubian could possibly be offset by raising everyone's headways from 6 min to 7.5 min. A lot of other expansions can be offset through the use of short-turns (Kenmore) and reverse-branching (sending Huntington trains to Seaport). But a key subset cannot be offset in this way: Nubian and anything that goes through a Comm Ave trunk -- Boston College, Oak Sq, Harvard, Grand Junction. Even at 10-minute headways (debatably low), those branches would already consume 30 of those 40 current tph slots. And even the bare essentials of these -- Harvard, Boston College, Nubian -- underserved at 10 minute headways would still eat up half of the current capacity.

You imply that with reduced transfers, Park St could increase its TPH by something close to 200 percent. But even more modest proposals we discuss here would require an increase starting at 130% of current capacity. Either 130% or 200% would both be pretty strong claims. I really want to believe that it's possible, but throwing confetti and waving hands hasn't convinced me. Until then, I'm going to continue to believe that some sort of Kenmore-to-South Station connection is well-worth consideration.
 

Riverside

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I may be totally missing something (and apologies if I am) but wouldn't this basically forfeit service to Copley and Arlington? How does a Cambridge/Somerville RL rider or Eastie/Revere/Lynn BL rider now get to the commercial high spine core at Copley?
Arlington I don't have a great solution for -- a Red Line rider would have the choice of a three seat ride (transfer at Park and again at Boylston), or a slightly out-of-the-way two-seat ride (transfer at South Station), while a Blue Line rider would be worse off -- either a three-seater via GC, or a ridiculous out of the way trip via Kenmore.

Copley, on the other hand, would be served by the north-south Green Line via stations at Back Bay and Prudential, with transfer available to the Red at Park and the Blue at GC. If someone needed to get to Back Bay proper (like Comm Ave at Dartmouth), they'd have a choice between the aforementioned paths to Arlington (plus one stop), or alternatively, they could take the Blue Line to some intermediate stop along the river -- maybe at Clarendon, and walk just about as far as they would from the current Copley station. Blue obviously would be a one-seat ride from Eastie, and a two-seat ride from Cambridge via Red transfer at Charles, or from Somerville via the north-south Green Line plus a transfer to Blue at GC or east-west Green Line at Boylston.

(If it were up to me, I'd rename this separated east-west Green Line to "Emerald Line," given that it parallels the Emerald Necklace so much.)
 

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