Green Line Reconfiguration

[TheRatmeister]

Great thoughts from you both. I'll address this part first:

The idea is to set up three completely isolated trunks:

• Nubian - Park Inner
• Huntington - Government Center
• Kenmore - Seaport

Of the three downtown routes, Park Inner is the only one with a capacity constraint (or at least the most likely). Coincidentally, of the three outbound routes, Nubian is the only one that doesn't need "full capacity" of 30-40 tph.

(You can swap the latter two to Huntington-Seaport and Kenmore-GC, which is equivalent and doesn't change the rest of this comment.)

As it turns out, this is the only arrangement that maximizes capacity everywhere without creating a "Herald Square-type merge". Why? The derivation of this is precisely what I'll elaborate in the next blog post.

So if you want three trunks you need two extra tracks between Boylston and Washington St, and that pretty much means doing some form of SL Phase 3 as the easiest link between Boylston Inner and Washington St is probably from the west... somehow. Of the portal alternatives:

• Columbus Ave is in completely the wrong place
• Tremont St is completely unworkable due to the insane loop you'd need at Boylston

Which leaves the Charles St and Stuart St Alts. They could both possibly work, not needing to go nearly as deep should alleviate at least some gradient concerns, but may make construction quite disruptive. Both of these options also involve destroying the buried Pleasant St portal and its flying junction however, which could complicate weaving the Huntington subway into all this.

 

[TheRatmeister]

And we could also go deeper with the "Why?" for the Nubian trunk in the first place. I don't think the corridor really has a lot of branching opportunities, and even with 12 TPH to Nubian, 12 to Needham/Riverside, and 6 to Hyde Sq or Arborway, that's still within the constraints of a single trunk.

 

[pfd12]

So MassDOT had a public hearing yesterday on the 75% design of the Webster Ave Bridge replacement project, by Union Square in Somerville. I joined late and missed the presentation (they'd already started the Q&A) so take this with a grain of salt - but it sounds like the proposed work might get in the way of a future GLX to Porter.

My understanding, based on this 2021 post and this 2013 post by @F-Line to Dudley, is that in order to be compatible, the Webster span would either need to either be kept the same (other than redoing/shifting supports for a pipeline attached to the bridge) or possibly even widened a few feet. However, the width under Webster is actually being made narrower. MassDOT (or more specifically the consultant) said that the new bridge foundations are being poured in front of the existing ones, therefore narrowing the width - but basically said this was OK because the end result wouldn't be narrower than the neighboring Prospect bridge.

On the other hand, someone who was there for the part of the meeting I missed told me that MassDOT had both (1) acknowledged that GLX to Porter was in the MBTA's long-term plans and (2) said that the MBTA had reviewed the bridge plans and said it did not conflict with their long-term plans. So while not exactly saying it directly, that seems to imply "this work will not preclude a future GLX extension."

Is there something I'm missing? If the horizontal clearance at Webster gets narrowed, then could the extension be done by tunneling through the bridge, just like would be needed at Prospect St?

Someone from STEP asked if the horizonal clearance at Webster could be widened instead of narrowed. The consultants said they looked into it, but due to some MWRA pipeline that was relatively recently installed/which would be too expensive to redo, a widening would require the bridge span to be 20' longer (so that the abutment could sit north of the pipeline). This would require additional depth (I'm not a civil engineer, not sure what depth means in this context) and therefore would require reconstructions for several hundred ft of the streets leading up to the bridge. They decided this was too disruptive and complicated, hence the approach of narrowing/having the new foundations in front of the existing ones.

 

[Riverside]

Finally circling back to some of this.

I'd guess that's much more feasible than turning 25-40 tph at Park St, simply because the Seaport Transitway has a lot more options on the eastern end to terminate trains.

Obviously I'm not a train operator myself, but AFAIK, a lot of the time-killers in terminal operations actually come from the "software" side: ensuring that everyone gets off, adjusting displays and announcements to change directions, etc. Seaport has at least two such terminals to do this: Silver Line Way and Design Center. And SLW makes it very easy to set up separate terminating vs. through-running eastbound platforms

Yes, good point. And I believe there's actually space for passing tracks at Courthouse as well, which could potentially be used for turning trains when needed.

In any case, I think the Criss Cross vs Tripod dynamic does elucidate underlying comparative strengths: Tripod may be viable, but ultimately it requires Park Street to absorb trains from three trunks: Kenmore, Huntington, and Nubian; Criss Cross requires it to absorb only two. Tripod limits Kenmore branches to whatever can fit in the Park St Inner Loop; Criss Cross provides a dedicated subway that's able to be fully isolated. Those dynamics stay true regardless of the Inner Loop's actual capacity.

I'm confused. Why should the total tph across all 4 tracks at Park St matter under Tripod, if, under regular operations, the two inner tracks and the two outer tracks can be largely independent of each other?

(Or, in other words: Assuming we can comfortably send 30 tph from Boylston Outer to Government Center, why should it affect those trains if we add another 10-20 tph from Boylston Inner to Park St Inner?)

One possible answer is signaling, but I remain unconvinced. The NYC subway, despite its notoriously old-school operational practices, has no trouble with two trains departing or entering a platform on express and local tracks respectively at the same time. Unless there's a need for a train to switch between the inner and outer tracks, I don't see why other trains at Park St can't do this.

I'm embarrassed to admit that I'm not 100% sure I remember the exact point I was getting at here. But I think I was making the (perhaps trivial) point that, if Outer Park capacity is x and Inner Park capacity is y, then Tripod provides Huntington + Nubian with a capacity of x while Criss Cross provides Huntington + Nubian with a capacity of x + y. So, x + y > x, regardless of any of the specific values.

I know what you mean here, but I'm always pretty skeptical when this line of reasoning is brought up. The tunnel's proximity to the "Main St" of a neighborhood doesn't matter: it's the station locations that matter. In this case, the only possible intermediate station would be some counterpart to the current Arlington station -- such as at Arlington St itself, or at Bay Village. Realistically, there are two potential station locations: something near Park Plaza, and something near the Pleasant St Portal. Park Plaza is obviously close to stuff, but it's also quite close to Arlington. Bay Village at least fills a gap in coverage (including providing better access to the South End on the other side of the Pike).[Am gonna break your reply up into bullet points for ease of response

For one, I imagine that if there's one crucial intermediate station that any "Essex Criss-Cross" or "Stuart-Kneeland Criss-Cross" will need, it's not Arlington -- but an interchange with the Orange Line, around either Chinatown or Tufts Medical Center. An Arlington replacement (or the ability to stop at present-day Arlington) pales in comparison.

It's true -- aside from a direct alignment under Essex St with a transfer at Chinatown, both Stuart/Kneeland and Bay Village provide unideal transfers to Orange. This is part of why I think having the crossover at Back Bay is better (to address a question raised by @TheRatmeister): a transfer at Back Bay is probably going to be better than transfers at either Bay Village or Stuart/Kneeland.

Let's also not forget that, especially for Criss-Cross, the goal of any intermediate stations is not so much to "add something onto the T map for the first time", but "as a replacement for Kenmore/B/C riders heading downtown".

This is a fair point.

Such riders treat the intermediate stations as destination nodes, whether they're for

(Brief comments on each of these)

Public Garden, ... shops at the eastern end of Newbury St, jobs at Park Plaza,

I'm always loathe to use this argument, but these are all basically a half-mile from Copley, a "Shawmut" station under Marginal Road, or even a new station at Back Bay. I hate using that argument when talking about disrupting/relocating existing services, so I think it's a weak argument here, but it does seem worth noting that we wouldn't otherwise blink at station spacing with these kinds of walks.

Boston Common

Yeah, this one hurts, there's no way to deny that.

Asian restaurants in Chinatown (including for the Asian populations living in Allston)

I'm skeptical that a Chinatown Gate station would be insufficient for this purpose. Boylston is just as much on the outskirts of Chinatown as Chinatown Gate (or even a "Leather District" station at Beach & Lincoln) would be, and a new station would have the advantage of serving the opposite end of Chinatown (as opposed to today's status quo, where both stations are on the west side of the neighborhood).

Tufts Medical Center

I think a "Shawmut" station would be pretty similar (if not maybe slightly better) to today's status quo on this point. Boylston is equally on the periphery of Tufts Medical, and a "Shawmut" station would at least have an in-station walking path to the center of the Center.

Stopping at Bay Village and maybe Chinatown Gate is a huge inconvenience for the first three, and a minor inconvenience for the last two.

Okay yeah, I do think this is a legitimate critique. A tunnel under Stuart Street would more nearly recreate the service that is being diverted. Topologically, I think Criss Cross is mostly agnostic to the Stuart vs Marginal alignment (as is Tripod), so I think the pieces can be (somewhat) disconnected into separate rather than sequential conversations.

Yes, having a Bay Village station really helps nearby residents, especially those south of the Pike. But the existence of a Bay Village station is practically guaranteed thanks to the Nubian branch, regardless of where the second east-west LRT trunk will be and which routes it serves. I doubt the benefits of additional service at Bay Village outweigh the inconvenience of riders who now need a transfer for their old commute patterns. (This discussion can still be had for Tripod and rerouting the D/E to Bay Village, but at least they would still stop at Boylston.

Yeah, that's a good point about a Nubian Branch providing service to Bay Village no matter what.

Interestingly, Tripod actually makes much better use of both flying junctions, particularly the Pleasant St incline, than Criss-Cross: ....

Haha, this was in the back of my mind when I wrote this, and I appreciate the irony. My comment was really directed at the Marginal vs Essex debate, as well as the question of how to bring Huntington trains into the Boylston Inner tracks.

The funny thing is that chasing after the maximal usage of both flying junctions was indeed one of the things that I saw as most elegant about Tripod. And I think that I nod to this in my graphics, where my notes on the Tripod graphic highlight the maximum flexibility and one-seat rides it offers. It's a legitimate pro of the Tripod model, for sure.

But yes -- even under Criss Cross, the Pleasant St Junction might be used if there end up being two branches to Nubian (e.g. a surface and a subway), or if the lines are built in stages. And I should also note that while I think Criss Cross's topology is operationally stronger as a default service pattern, I think building a non-revenue Marginal <> Tremont connector could still be valuable under Criss Cross, particularly for accommodating service diversions or Red Sox/Celtics games/Garden events.

[more shortly, I hope]

 

[Riverside]

(Ugh, the forum software ate my quote linking.)

@TheRatmeister said:

Beware of sunk-cost fallacy. The existence of those flying junctions gives us great options to build new transit going forward, but it should not dictate what that transit looks like.

I agree with the principle. I wish I had a firmer sesnse of the cost of a new flying junction, compared to basic C&C (or even just flat tunnel borings).

@Teban54 said:

The idea is to set up three completely isolated trunks:

• Nubian - Park Inner
• Huntington - Government Center
• Kenmore - Seaport

Of the three downtown routes, Park Inner is the only one with a capacity constraint (or at least the most likely). Coincidentally, of the three outbound routes, Nubian is the only one that doesn't need "full capacity" of 30-40 tph.

(You can swap the latter two to Huntington-Seaport and Kenmore-GC, which is equivalent and doesn't change the rest of this comment.)

As it turns out, this is the only arrangement that maximizes capacity everywhere without creating a "Herald Square-type merge". Why? The derivation of this is precisely what I'll elaborate in the next blog post.

Without knowing the full extent of Teban54's argument, I'll just note that I agree that it would be parsimonious (in many ways) for Nubian to be able to use the Park St Inner tracks on a dedicated basis.

@Teban54 (and indirectly responding to some of @TheRatmeister's points), if we're just sketching lines on a map, two thoughts come to mind. Both involve surface-running the Nubian Branch up Arlington St and using the wider segment of Boylston along the Public Garden to (ironically) recreate the portal that was once there.

Option A uses most of the existing subways, plus a Marginal Subway, plus a Kneeland Subway accessed via Charles St, and a new transfer station at Public Garden (having closed Arlington):

Option B is a lot more construction heavy, using a full-length Stuart-Kneeland Subway, but has better transfers (IMO). Image isn't inserting but I think is attached. Hollow platform rectangles indicate surface stations. Alignment via Charles St here, but I guess you could dig up Columbus and put a portal there, to keep the curves a little less aggresive.

In both options, I think you could also add a subway branch to Nubian, indicated with dashed lines here.

The added street running for the surface Nubian Branch is annoying, but if it's isolated to the Inner tracks, it wouldn't create cascading disruptions. And if it's already going to be 1.8 miles of surface-running, 2.2 miles doesn't seem like it would break the bank. (It wouldn't be my first choice, of course. I guess you could also do something like Option B, but with Surface Nubian tunnel portaling down along the Mass Pike at Herald St, underpin the new station under Tremont St and then cut up Charles St.)

 

[TheRatmeister]

(Ugh, the forum software ate my quote linking.)

@TheRatmeister said:

I agree with the principle. I wish I had a firmer sesnse of the cost of a new flying junction, compared to basic C&C (or even just flat tunnel borings).

@Teban54 said:

Without knowing the full extent of Teban54's argument, I'll just note that I agree that it would be parsimonious (in many ways) for Nubian to be able to use the Park St Inner tracks on a dedicated basis.

@Teban54 (and indirectly responding to some of @TheRatmeister's points), if we're just sketching lines on a map, two thoughts come to mind. Both involve surface-running the Nubian Branch up Arlington St and using the wider segment of Boylston along the Public Garden to (ironically) recreate the portal that was once there.

Option A uses most of the existing subways, plus a Marginal Subway, plus a Kneeland Subway accessed via Charles St, and a new transfer station at Public Garden (having closed Arlington):
View attachment 61286

Option B is a lot more construction heavy, using a full-length Stuart-Kneeland Subway, but has better transfers (IMO). Image isn't inserting but I think is attached. Hollow platform rectangles indicate surface stations. Alignment via Charles St here, but I guess you could dig up Columbus and put a portal there, to keep the curves a little less aggresive.

In both options, I think you could also add a subway branch to Nubian, indicated with dashed lines here.

The added street running for the surface Nubian Branch is annoying, but if it's isolated to the Inner tracks, it wouldn't create cascading disruptions. And if it's already going to be 1.8 miles of surface-running, 2.2 miles doesn't seem like it would break the bank. (It wouldn't be my first choice, of course. I guess you could also do something like Option B, but with Surface Nubian tunnel portaling down along the Mass Pike at Herald St, underpin the new station under Tremont St and then cut up Charles St.)

I'm reasonably confident at this point that the Essex St alignment is the easiest connection between the Boylston St Subway and South Station. There is a clear spot for a flying junction west of Charles St, and there shouldn't be any unreasonable grades. I also think if you're fine severing the connection between the Boylston and Tremont St subways, it should be possible to tie the Pleasant St Portal into the inner tracks at Boylston by digging a shallow subway under Charles St, hopefully just grazing the top of the diving Essex St tunnel before making a sharp right turn to meet the existing subway into Boylston. However, I think that last part is only necessary if you want to add more branches to Huntington/Nubian service, and I don't really see a lot of benefit to doing that. The Boylston St subway definitely could use additional branches however, sending the A/B/C into Park St, and a Harvard/Brookline Village (and maybe even rush hour Riverside/Needham services) to Seaport.

Here's a rather crude map of what the "base" buildout could be: (so crude it breaks my rule on using Silver as a color, I'll pick a new one later.)

 

[vanshnookenraggen]

I'm reasonably confident at this point that the Essex St alignment is the easiest connection between the Boylston St Subway and South Station. There is a clear spot for a flying junction west of Charles St, and there shouldn't be any unreasonable grades. I also think if you're fine severing the connection between the Boylston and Tremont St subways, it should be possible to tie the Pleasant St Portal into the inner tracks at Boylston by digging a shallow subway under Charles St, hopefully just grazing the top of the diving Essex St tunnel before making a sharp right turn to meet the existing subway into Boylston. However, I think that last part is only necessary if you want to add more branches to Huntington/Nubian service, and I don't really see a lot of benefit to doing that. The Boylston St subway definitely could use additional branches however, sending the A/B/C into Park St, and a Harvard/Brookline Village (and maybe even rush hour Riverside/Needham services) to Seaport.

Here's a rather crude map of what the "base" buildout could be: (so crude it breaks my rule on using Silver as a color, I'll pick a new one later.)

View attachment 61288

This is the first time I've seen a fantasy MBTA map here that I think is actually good. It's also really well drawn.

I'd argue the C should replace the Shuttle to SBW, but the overall idea is sound.

 

[TheRatmeister]

This is the first time I've seen a fantasy MBTA map here that I think is actually good. It's also really well drawn.

I mean it's just my most recent fantasy map but very lightly modified and with everything that's not GLRC cut out. I'd not really consider either of them to be great pinnacles of design, if you want that I'm still very proud of my current map redesign.

 

[Teban54]

I'm reasonably confident at this point that the Essex St alignment is the easiest connection between the Boylston St Subway and South Station. There is a clear spot for a flying junction west of Charles St, and there shouldn't be any unreasonable grades. [...]

Not trying to nitpick, but keep in mind that Essex St is still much narrower than Stuart/Kneeland, and even if the tri-level Chinatown station is feasible, it will still be incredibly costly.

Stuart-Kneeland offer you a much wider street to dig under (possibly even allowing C&C), a more conventional and thus cheaper transfer station to the Orange Line, and less engineering hassle to cross under OL at a point with two platforms and two OL tracks of different elevations.

 

[TheRatmeister]

and even if the tri-level Chinatown station is feasible,

We know from SL Phase 3 that it is. While that part does require some steeper grades to hook into the Transitway, the EIS doesn't raise any concerns about that part, which suggests that part of the project was pretty solid in terms of engineering, it's only once you start trying to connect it to Washinton St that things get really questionable. The EIS from 2005 is available online in full, and that part really doesn't have anything dubious or challenging, it's a straight shot . The portals and especially the loop however, that's questionable and I'm not optimistic about those being feasible as planned. (Hence why I'm not looking at using them.) There isn't a good chart showing what the max grade for the buses on this segment would be, but it seems to be about 5%, maybe 5.5% over an 800-1000ft stretch. The Green Line already has a segment with a grade of 6.5% over a similar length, so I'm not terribly concerned about this being a problem.

Stuart-Kneeland offer you a much wider street to dig under

It's also a longer route. Around 25% longer for Boylston St-South Station. So which costs more, 25% more tunnel or a two-level station? I don't personally know, but I wouldn't be surprised if it's pretty similar, in which case the better transfer from Essex St would win.

You also don't totally avoid the narrow streets, you've got to hook into the connector at Essex St somehow, so you'd need either the surface connector or a tunnel under one of the other nearby narrow streets such as Lincon St or South St.

(possibly even allowing C&C)

Given that you need to get under both OL tunnels and the Tremont St subway, I'm quite doubtful that would be feasible. This is also an old part of Boston, actually within the original boundary of the shoreline, not fill. I can't imagine it would work out cheaper. The depth also seems like a stretch for C&C, the OL at Chinatown is apparently 21ft below the surface according to the SL Phase 3 plans, and not significantly shallower south of the station, so you'd probably be below 30 feet, possibly more like 40 or 50ft, for any new tunnel.

a more conventional and thus cheaper transfer station to the Orange Line

More conventional, maybe. Cheaper? Possibly, butyYou're further away from the OL platforms, so new pedestrian tunnels are needed, adding additional cost. Better? Definitely not, it would be more akin to the dreaded transfer at State. It also lacks an easy connection to Boylston, so for service 2SR or better service between Seaport and Huntington it either requires the Charles St connector with a new flying junction for through service, or maze-like tunnels going above the Stuart St subway but below the Tremont St subway to Boylston. (Or I guess the opposite, with the pedestrian tunnels being below both subways but that seems like more work for no benefit.)

Stuart-Kneeland offer you a much wider street to dig under (possibly even allowing C&C), a more conventional and thus cheaper transfer station to the Orange Line, and less engineering hassle to cross under OL at a point with two platforms and two OL tracks of different elevations.

Platforms maybe not (but again this makes the transfer worse), but two elevations yes. You can see on the track chart that between Chinatown and Tufts the OL tracks are still at different heights.

 

[Riverside]

so crude it breaks my rule on using Silver as a color, I'll pick a new one later.

You could just use Copper for both, since they share the trunk along Boylston Street.

Beware of sunk-cost fallacy. The existence of those flying junctions gives us great options to build new transit going forward, but it should not dictate what that transit looks like.

I want to come back to this, and I guess maybe briefly step back and talk about objectives here. For me, the GLR concept is not exactly about figuring out what transit should look like. Rather, I see two objectives in parallel:

1) Identify a viable design that achieves basic objectives. By "viable", I mean "physically viable" and "financially viable insofar as reduced costs can be targeted through lower-risk builds". I say "lower-risk" because I'm not a professional -- I don't actually have a way of assessing the viability of these proposals in absolute go/no-go terms. So my goal is to find low-risk options for as many components as possible, and identify the strongest possible cases to justify the higher risk components. "Viability" does not mean "addresses all drawbacks"; it just means achieving basic objectives (including integrated subway service to the Seaport, better transit to Longwood, and increased capacity and reliability systemwide) in a way that minimizes costs as much as possible.

2) Identify options and alternatives for enhancements. With a basic MVP identified, we can then highlight options for higher-spend builds, articulating the pros and cons for each. Critically though, I don't see a way (or reason) to identify the single best alternative*. A host of factors -- geological, physical, political, financial, social, community, operational -- will dictate what the best alternative will be, and so it's an unanswerable question.

* (unless we choose to operate purely hypothetically with no regard for cost; as I've mentioned elsewhere, I am a huge proponent of this approach as I think it's a vital part of the conversation, but it needs to be clearly articulated when we are in the "no costs mode". [The other side of that coin is to operate in "Robert Moses mode" and I doubt any of us have interest in doing that either.])

Tripod and Criss Cross both achieve minimum viability, from what I can tell. They both have pros and cons. I prefer Criss Cross, but ultimately choosing the best option of the two is unanswerable. Criss Cross creates an Allston <> Seaport 1SR but loses the Allston <> Boston Common 1SR. Tripod creates a Huntington <> Seaport 1SR, but rules out a resurrected A Branch through the Central Subway. Absent formal study and community engagement, there's no way to meaningfully say which of those options would be best; it's only possible to note the knowable pros and cons. That's what I tried to do in my graphics a couple pages back.

Both Tripod and Criss Cross have options for enhancement. As @Teban54 has laid out, a connection between Arlington Station and Bay Village to create the "Full Criss Cross" would enable Seaport 1SRs for both Allston and Huntington. The cost is increased operational complexity, and more complex tunneling (including some of the more invasive modifications of the current Central Subway that have been discussed), in turn increasing cost.

Re-reading my reply from earlier this weekend, I think I lost sight of these goals a little bit. @Teban54 is right: Criss Cross disrupts existing Allston <> Arlington journeys, with modest but insufficient mitigation. That's pretty straightforwardly a con, and I should have just agreed. There will be no perfect proposal raised through discussion on ArchBoston and we should be okay with that (and skeptical of ourselves when we think we've miraculously generated a seemingly-perfect proposal).

"Full Criss Cross" addresses the needs of a larger scope: not merely "integrated subway service to the Seaport" but "integrated subway service to the Seaport with no disruption to existing 1SRs" or "subway service to Seaport with 1SRs from Kenmore and from Huntington". If that larger scope is a must-have, then Tripod and Criss Cross are not fit for purpose, and that's fine.

Speaking purely personally, I'm less interested in solving that larger scope using the Green Line Reconfiguration approach, so while obviously everyone else should continue to discuss that as much as they want, I probably won't participate in that aspect as much.

And circling back to this:

Beware of sunk-cost fallacy. The existence of those flying junctions gives us great options to build new transit going forward, but it should not dictate what that transit looks like.

What I think transit should look like in this domain is closing the Mass Pike within 128 and turning it entirely over to transit, with a line that picks up the Transitway to go through the Seaport and across to the airport before turning north toward Chelsea and maybe Everett. Then build a new heavy rail subway from Nubian (and/or points south) through the South End (probably under Harrison) to South Station, Congress St, and North Station, before turning northeast to serve Chelsea and Revere. Put Huntington into a subway that runs under Longwood and then out to Riverside and Needham, with a spur built into JP to Hyde Square, connecting to the Central Subway via Pleasant Street. And I would reroute the Red Line to serve the Seaport more directly.

None of that achieves the goals that I see as specific to the Green Line Reconfiguration concept. I'll raise it in Fantasy T Maps, God Mode, or Crazy Transit Pitches, but I see it as out of scope for this particular discussion.

 

[TheRatmeister]

Would anyone here know where I could find details about the proposed grades for the "Core segment" of SL Phase III? I think the Essex St alignment was settled on in 2002ish, anything from then? The 2005 EIS has loads of detail about the portals but relatively little about the core section. The one map that shows grades, the soil layers chart, suggests a 10% gradient which can't possibly be right, unless I'm reading the chart completely wrong.

 

[Tallguy]

Would anyone here know where I could find details about the proposed grades for the "Core segment" of SL Phase III? I think the Essex St alignment was settled on in 2002ish, anything from then? The 2005 EIS has loads of detail about the portals but relatively little about the core section. The one map that shows grades, the soil layers chart, suggests a 10% gradient which can't possibly be right, unless I'm reading the chart completely wrong.

This seems more of a God mode posting

 

[TheRatmeister]

I've gone back for another look and I think I have at least a rough estimate. The main limiting factor for gradient on the South Station incline is the New East Side Interceptor sewer, around 25ft below the surface. The tunnels need to rise around 16-17 feet to meet the Transitway at ~8-9ft depth. Over a horizontal distance of about 320ft, that corresponds to a gradient of about 5.5%, depending on the exact distance. I am therefore not particularly concerned with this part of the design.

The other incline needed for a new Phase III (or really just a new project at this point that's based on Phase III), would be between Arlington and Boylston. I can't tell if there are any relevant major sewers west of Charles St, but east appears to be clear. The question here is if you can dive under a connection at Charles St to allow for a "full" criss-cross with both the Nubian branches continuing to the inner tracks. My answer there is also yes but a bit more hesitantly as I'm not sure where the descent would actually start. It seems like you need to descent about 30 feet before reaching Charles St to stay in the clear, so 280 Boylston St or further would keep that to a 5% grade.

The main problem I can see currently is the stacked platform design. Any station west of Boylston St cannot have stacked platforms with this design, you'd need a grade above 7%. The Tremont St alt. does feature a tunnel underpinning the Green Line, so maybe this isn't an issue? At Chinatown, the design does not underpin any of the surrounding buildings and thus stacked platforms are needed, the deeper of the two being ~78ft below Essex St. To use this design, it is at minimum required to have a non-stacked station at Boylston. If that's not possible, the mid-block alternative would need to be re-examined. I strongly suspect that it's not as bad as it was presented in the report. Their primary objection of a grade exceedance related to the portals which would not be used as part of this alignment, so that concern is automatically dealt with. Their other main concern was about long dwells, potentially exceeding headways. I believe this is not a problem if you don't intend to run 74?!?!?! vehicles per hour through the stations. (One bus every 48 seconds.) Copley manages to function with a ridership comparable to Boylston+Chinatown put together, so I think it's doable. The transfer quality isn't that bad, at around a 400ft walking distance from the inbound platform at Boylston it would be about a 1.5-2 minute transfer. If this is truly catastrophic some moving walkways can be installed or something.

So, TL;DR to answer @Tallguy's question, I think it's feasible.

 

[Teban54]

I've gone back for another look and I think I have at least a rough estimate. The main limiting factor for gradient on the South Station incline is the New East Side Interceptor sewer, around 25ft below the surface. The tunnels need to rise around 16-17 feet to meet the Transitway at ~8-9ft depth. Over a horizontal distance of about 320ft, that corresponds to a gradient of about 5.5%, depending on the exact distance. I am therefore not particularly concerned with this part of the design.

The other incline needed for a new Phase III (or really just a new project at this point that's based on Phase III), would be between Arlington and Boylston. I can't tell if there are any relevant major sewers west of Charles St, but east appears to be clear. The question here is if you can dive under a connection at Charles St to allow for a "full" criss-cross with both the Nubian branches continuing to the inner tracks. My answer there is also yes but a bit more hesitantly as I'm not sure where the descent would actually start. It seems like you need to descent about 30 feet before reaching Charles St to stay in the clear, so 280 Boylston St or further would keep that to a 5% grade.

The main problem I can see currently is the stacked platform design. Any station west of Boylston St cannot have stacked platforms with this design, you'd need a grade above 7%. The Tremont St alt. does feature a tunnel underpinning the Green Line, so maybe this isn't an issue? At Chinatown, the design does not underpin any of the surrounding buildings and thus stacked platforms are needed, the deeper of the two being ~78ft below Essex St. To use this design, it is at minimum required to have a non-stacked station at Boylston. If that's not possible, the mid-block alternative would need to be re-examined. I strongly suspect that it's not as bad as it was presented in the report. Their primary objection of a grade exceedance related to the portals which would not be used as part of this alignment, so that concern is automatically dealt with. Their other main concern was about long dwells, potentially exceeding headways. I believe this is not a problem if you don't intend to run 74?!?!?! vehicles per hour through the stations. (One bus every 48 seconds.) Copley manages to function with a ridership comparable to Boylston+Chinatown put together, so I think it's doable. The transfer quality isn't that bad, at around a 400ft walking distance from the inbound platform at Boylston it would be about a 1.5-2 minute transfer. If this is truly catastrophic some moving walkways can be installed or something.

So, TL;DR to answer @Tallguy's question, I think it's feasible.

Great thoughts overall. A few somewhat specific questions and comments:

• Can you point out exactly where the New East Side Interceptor sewer is? Is it running down the Big Dig / Surface Rd?
  • (If that's indeed the case, then I agree that this factor shouldn't be a deal-breaker at all -- any proposal to extend the Seaport Transitway would then be required to descent this much, likely even including those that turn at Lincoln St. The only alternative that avoids it is turning at South St, which has its own share of issues.)
• The main concern with dwell time is that a combined Boylston-Chinatown station would have too much ridership, due to absorbing all transfers from both the Tremont St subway and the Orange Line, plus lots of downtown traffic entering from street level. This is even worse if you don't do Full Criss Cross. It's one of the main reasons why "combining RL Park St and Downtown Crossing" is a bad idea.
• Were you able to find anything about the cost of an Essex St subway, specifically due to building mitigation for tunneling under a narrow street?
  • The Boylston-Essex corridor narrows down to 40 ft building-to-building at various stretches, typically skyscrapers. @F-Line to Dudley had always insisted that tunneling under narrow streets abutting tall buildings like this, including Longwood Ave, is a straight-up no.
  • Even though today's Green and Orange lines are under similarly narrow downtown streets, I'm not sure if building a brand new tunnel -- after the building foundations already exist -- changes things significantly. Intuitively, that sounds plausible.

 

[TheRatmeister]

Can you point out exactly where the New East Side Interceptor sewer is? Is it running down the Big Dig / Surface Rd?

Under South Street, at least where it intersects Essex St. This would suggest to me that any alignment using Lincoln St, Hudson St, etc would still encounter this problem. The only way around it would be a surface connector. The planners of SL Phase 3 of course knew this, and seemed to be fine with that short section of relatively steep grade.

The main concern with dwell time is that a combined Boylston-Chinatown station would have too much ridership, due to absorbing all transfers from both the Tremont St subway and the Orange Line, plus lots of downtown traffic entering from street level.

I mean yes, of course. But my point is that Copley functions with a similar number of riders as Boylston+Chinatown combined today, and Park St+Govy manage as well despite having very high transfer volumes. The amount of people transferring to Tremont St would also likely be reduced by the existence of a Brookline Village branch, which would take most of the Longwood passengers. In the study they argued that the station would need to be wider to accomodate larger passenger volumes, which would require underpinning buildings, but I really don't buy that argument. Chinatown is going to be by far the bigger transfer station, you can see it in the SL5 ridership data today, so I have to imagine any Chinatown station would either run into these issues as well, or neither one would. Regardless, I think it's worth another look given that the main concerns related to loops and portals are out of the question, that was the biggest problem.

Were you able to find anything about the cost of an Essex St subway, specifically due to building mitigation for tunneling under a narrow street?

As a contract was never released I don't think there's any itemized breakdown of costs.

@F-Line to Dudley had always insisted that tunneling under narrow streets abutting tall buildings like this, including Longwood Ave, is a straight-up no.

Maybe they can interject with their thoughts, but I was under the impression that was mainly about C&C tunnels. I have to imagine it's designed as deep as it is to avoid the foundations, and thus any major problems from the narrow street.

 

[F-Line to Dudley]

Maybe they can interject with their thoughts, but I was under the impression that was mainly about C&C tunnels. I have to imagine it's designed as deep as it is to avoid the foundations, and thus any major problems from the narrow street.

SL Phase III's tunnel bore interacted with building foundations at both Chinatown and Boylston stations, adding a lot of mitigation cost to the core segment. The project's tripling price tag between 2003-2009 wasn't all tied up in the flailing over portal locations; a good portion of it was the escalating mitigations on the mostly-settled core alignment. In both station cases the building foundations narrowly breached the level of the upper/outbound bores. Not by much--it was a scrape--but it happened and building-by-building across the Essex-Boylston corridor we don't know what the depths are on other buildings that veer very close to the tunnel.


Chinatown:

Boylston:

It's hard because these close-abutting foundations are fairly deep, and there are clearly some buildings on the corridor that go >30 ft. deep. Increasing the tunneling depth is only going to increase the grades already so close to the max limit for LRT, so that's not an unlimited solution either. The Phase III project as designed maxed out just about all the possible wiggle room on the core alignment, leaving little left to chance.

 

[JeffDowntown]

SL Phase III's tunnel bore interacted with building foundations at both Chinatown and Boylston stations, adding a lot of mitigation cost to the core segment. The project's tripling price tag between 2003-2009 wasn't all tied up in the flailing over portal locations; a good portion of it was the escalating mitigations on the mostly-settled core alignment. In both station cases the building foundations narrowly breached the level of the upper/outbound bores. Not by much--it was a scrape--but it happened and building-by-building across the Essex-Boylston corridor we don't know what the depths are on other buildings that veer very close to the tunnel.


Chinatown:
View attachment 61417

Boylston:
View attachment 61418

It's hard because these close-abutting foundations are fairly deep, and there are clearly some buildings on the corridor that go >30 ft. deep. Increasing the tunneling depth is only going to increase the grades already so close to the max limit for LRT, so that's not an unlimited solution either. The Phase III project as designed maxed out just about all the possible wiggle room on the core alignment, leaving little left to chance.

There is a pretty big question raised by those designs. If you are width constrained, why incorporate the vertical access into the width dimension. Why not place vertical access at the ends of platforms, so the access does not add to the overall width? Or were there elevators at the ends so the other access needed to go mid-platform?

 

[F-Line to Dudley]

There is a pretty big question raised by those designs. If you are width constrained, why incorporate the vertical access into the width dimension. Why not place vertical access at the ends of platforms, so the access does not add to the overall width? Or were there elevators at the ends so the other access needed to go mid-platform?

There were elevators not pictured in those renders, and the egress location in general would have limited wiggle room because of the need to interface with the legacy Green and Orange transfer stations from the mezzanines. So there probably was not any practical option to place the egresses only at the extreme ends to save space. Being very busy transfer stations, it's also possible that there needs to be multiple egresses of each type which further limits the options for linking to the position-constrained connecting mezzanines.

EDIT: Yeah, impossible. Look where Chinatown's pair of egresses needed to be placed to interact with the Orange connections (especially with Orange having no consolidated mezzanine). . .

 

[TheRatmeister]

Increasing the tunneling depth is only going to increase the grades already so close to the max limit for LRT, so that's not an unlimited solution either.

The mid-block station actually gives a bit more wiggle room. You've got about 1700ft of slope on the shorter Boylston side, at a 5% grade that's 85ft of depth, plus the 20 from the existing tunnel depth gets you to a floor of 105' and a ceiling of about 80'. If that's deep enough to avoid any issues with building foundations, you might not even need the stacked station at Chinatown. That would free up the whole upper level for a mezzanine and pedestrian connection to Boylston. If we're feeling really brave, and width really isn't a problem, maybe we could even do Spanish solution platforms à la Park St to alleviate some capacity concerns.

Maybe that's not possible, but my point is that not being tied to the stupid loop at Boylston opens up some new possibilities, and I think they're worth exploring. As for costs, well that's going to be a problem no matter what until the MBTA learns how to effectively manage a project.