Green Line Reconfiguration

[Brattle Loop]

Maybe I'm misunderstanding, but I think this doesn't answer my question. For the purpose of this discussion, I'm looking for an alignment that allows the Green Line to remain underground north of the Pike, cross the Pike underground to the south, and also enable an underground subway east along Marginal toward South Station. I don't want a portal north of the Pike, and I think that is what you're describing? My apologies if I'm misunderstanding.

I think it's south of the Pike/NEC that's being described, because the NEC's never along Marginal, but does distinctly pull away from Herald Street in the vicinity of the Shawmut and Washington bridges. That was always my understanding of the area F-Line was referring to in discussing a southern portal and incline for Nubian service.

 

[Riverside]

Ohhhh I see I see, okay yes, that makes more sense. Hmm. The pullout doesn't start until ~100' west of the Shawmut bridge, at which point the Orange Line is still fully underground. So the Nubian Branch would need to be at Level -3 where it crosses the Orange Line, and then rise up to street level (Level 0) by the time it hits Washington St. That's a three level rise over a distance of about 600 feet (depending on where you put the crossunder). From our discussions above, let's assume that a level is 18 feet. 18 times 3 is 54 feet. So a 54 foot rise over 600 feet yields a grade of 9%.

From my read of the 1999 edition of the TCRP Track Design Handbook for Light Rail Transit, page 73 in the PDF (if someone knows a later edition, let me know), 7% is the steepest recommended grade, as long as (I think) the incline is less than 500' long.

If the Nubian Subway swings wider, and crosses under the Orange Line further to the west, you can give yourself 900' feet between the crossunder and Washington, which would yield a 6% grade (and probably faster speeds since the curve from a station under Tremont would be more gradual).

Okay so, yes, that could be feasible. Plus, since the Nubian Subway would need to be at Level -2 to cross under the Pike, that would leave Level -1 free for a shallow subway under Marginal for a Back Bay-South Station tunnel. That might point to a station at Tremont that is double-stacked instead of double-wide (with Nubian on the lower level at -2 and Back Bay service at -1 to hook into the Marginal Subway) but that is probably doable.

I really need to post my diagrams for this...

 

[Tallguy]

Dont forget NSRL tunnels

 

[F-Line to Dudley]

Dont forget NSRL tunnels

NSRL tunnel portal goes on the Washington-Harrison block. So anything crossing under would have to be west of the Washington St. overpass.

 

[themissinglink]

If the portal for the Nubian Sq branch was to be south of the Pike and the NEC, would this be a feasible alignment?

 

[Riverside]

Yes I think so. BUT. You will want to leave provisions for a connection between Boylston and Back Bay, and for a connection between Back Bay and the Seaport. I just spent way more time than I want to admit gaming that out, so I'll post those later tonight.

 

[Teban54]

Dont forget NSRL tunnels

Speaking of NSRL, is there any chance for the project to have provisions for a Bay Village-South Station GL tunnel that connects to the Transitway?

 

[F-Line to Dudley]

Speaking of NSRL, is there any chance for the project to have provisions for a Bay Village-South Station GL tunnel that connects to the Transitway?

No. The alignments...and especially depth...are totally different.

 

[Riverside]

Alright. Let's take a look.

Putting the Nubian portal just south of the NEC is a pretty elegant solution. You need to do some significant climbing and diving to go from street level to sub-Mass Pike to Tremont Street Bellmouth, but it should be doable.

If you want to future-proof for a subway under Marginal, you need to do some provisioning.

To review my "Level" nomenclature/shorthand: Level 0 is street-level, Level 1 is one "story" sub-surface (bottom of tunnel is 17-20 feet below pavement of street) and is the level that the Mass Pike runs at; Level 2 is another level below that, a.k.a. the depth of the Orange Line just under the Pike; Level 3 is one more, i.e. the depth of a tunnel that goes underneath the Orange Line underneath the Mass Pike.

Diagram below, but the highlights are:

• Build your Bay Village Park-Nubian platforms under Tremont Street at Level 2.5 on the western half of the street
  • to reach this depth from the bellmouth should require about 400 feet of running track, which just about fits;
  • the southern end of your platform should leave enough running space for the track to descend 0.5 Levels before hitting Marginal Street -- should be about 130 feet, which just barely fits;
  • by now you are at Level 3 and clear to go under both the Pike and the Orange Line
  • It will be tempting to start journeying under the Pike at a shallower depth, i.e. Level 2, and desend under the Pike to reach Level 3. In my opinion, the better option is to reach Level 3 under Marginal Street -- see below
  • [EDIT to add: you will also want to "swing wide" when tunneling under the Pike -- should be pretty much aligned with a western platform, but you will need the full 130 feet (if not a little extra) in order to descend to Level 3 before Marginal]
• When it comes time to expand Bay Village station to include Marginal Street services to Back Bay and the Seaport, double-stack your platforms at Levels 1.5 and 2.5 on the eastern half
  • I think you are better off putting Park-bound service on the lower level
  • (This also means that all Park-bound service can leave from the same platform on two tracks)
  • Strictly speaking, you don't have to superimpose your platforms on top of each other; they could be off-set -- the main purpose to the grade separation is to provide vertical distribution for a 3-way flying junction south of the station -- the horizontal distribution is less critical
• Whichever leg you build first -- to Back Bay or to Seaport -- make sure to leave enough space for a complementary flying junction on the opposite side
  • You will need about 260 feet of running track between your eastern junction and your western junction, as the eastbound and westbound tracks will both need to switch to the opposite level they enter the junction from
  • Both your Level 1.5 track and your Level 2.5 track will rise as they exit Bay Village to hit Level 1 and Level 2 respectively; this is how you will fit your Nubian tunnel underneath an east-west Back Bay-Seaport subway under Marginal, and is why your Nubian tunnel needs to descend to Level 3 before hitting Marginal
  • To rise that 0.5 Levels, your tracks will need 130 feet of running space; that is easy to get on the Back Bay leg, but will require a tight turn on the Seaport leg
• Finally, when building your subway to the Seaport, I suggest provisioning a gap northeast of the Harrison/Hudson intersection (currently a parking lot); this future-proofs a Nubian-Seaport leg, either descending from a surface portal, or ascending from a subsubway that has come up under the Pike

Here is a crude diagram of what I'm describing with full build at Bay Village:

Like I said, you don't have to put the Back Bay/Seaport track directly above the others -- you could off-set it to the western side. (That decision will be partially impacted by the concourse design for the new station, which I didn't really game out the full details of.)

And here is the diagram. Hopefully self-explanatory. Darkness of the coloring corresponds to tunnel depth -- Level 1, Level 2 and so on. The effort at depicting the superimposition of the platforms is not great, but hopefully clear enough.

And zoomed in. The exact measurements of the grades -- especially in the junction -- are not precise, and so the alignments may need adjusting. But there should be space for everything to fit.

Under the theory that the Nubian Branch will get built first, practically speaking this really boils down to one thing: plunk your Park-Nubian platform at Level 2.5, and if possible, dig to Level 3 before hitting Marginal.

That being said... as you can see, the envelope of the station is such that a Nubian-Park platform built at Level 1.5 could then subsequently become a lobby in a future expanded station. You could even probably carry out the work underneath while keeping the station open above, and with good staging could reduce the need for service closures. Once the lower level of the station is built you would need to lower the tunnels to the north and south, which would disrupt service but possibly could be done without excavating the entire street a second time.

And in the (highly unlikely) scenario where either a Back Bay or Seaport Branch gets built first, stacking the platforms still gives you the most flexibility for adding the second leg later on, and potentially can reduce the on-street impact of construction, due to the narrower profile.

 

[themissinglink]

View attachment 22850

Great artwork and explanation.

The only thing that I'd change would be the portal location (specifically the incline to street level) for the Nubian Square branch. The portal should probably be located on Washington Street immediately south of the intersection with Herald Street. This would be to ensure that the tracks are below grade through the Mass Pike Air Rights Parcels 20 & 21 in anticipation of potential future air rights development on those parcels.

 

[Teban54]

Alright. Let's take a look.

Putting the Nubian portal just south of the NEC is a pretty elegant solution. You need to do some significant climbing and diving to go from street level to sub-Mass Pike to Tremont Street Bellmouth, but it should be doable.

If you want to future-proof for a subway under Marginal, you need to do some provisioning.

To review my "Level" nomenclature/shorthand: Level 0 is street-level, Level 1 is one "story" sub-surface (bottom of tunnel is 17-20 feet below pavement of street) and is the level that the Mass Pike runs at; Level 2 is another level below that, a.k.a. the depth of the Orange Line just under the Pike; Level 3 is one more, i.e. the depth of a tunnel that goes underneath the Orange Line underneath the Mass Pike.

Diagram below, but the highlights are:

• Build your Bay Village Park-Nubian platforms under Tremont Street at Level 2.5 on the western half of the street
  • to reach this depth from the bellmouth should require about 400 feet of running track, which just about fits;
  • the southern end of your platform should leave enough running space for the track to descend 0.5 Levels before hitting Marginal Street -- should be about 130 feet, which just barely fits;
  • by now you are at Level 3 and clear to go under both the Pike and the Orange Line
  • It will be tempting to start journeying under the Pike at a shallower depth, i.e. Level 2, and desend under the Pike to reach Level 3. In my opinion, the better option is to reach Level 3 under Marginal Street -- see below
  • [EDIT to add: you will also want to "swing wide" when tunneling under the Pike -- should be pretty much aligned with a western platform, but you will need the full 130 feet (if not a little extra) in order to descend to Level 3 before Marginal]
• When it comes time to expand Bay Village station to include Marginal Street services to Back Bay and the Seaport, double-stack your platforms at Levels 1.5 and 2.5 on the eastern half
  • I think you are better off putting Park-bound service on the lower level
  • (This also means that all Park-bound service can leave from the same platform on two tracks)
  • Strictly speaking, you don't have to superimpose your platforms on top of each other; they could be off-set -- the main purpose to the grade separation is to provide vertical distribution for a 3-way flying junction south of the station -- the horizontal distribution is less critical
• Whichever leg you build first -- to Back Bay or to Seaport -- make sure to leave enough space for a complementary flying junction on the opposite side
  • You will need about 260 feet of running track between your eastern junction and your western junction, as the eastbound and westbound tracks will both need to switch to the opposite level they enter the junction from
  • Both your Level 1.5 track and your Level 2.5 track will rise as they exit Bay Village to hit Level 1 and Level 2 respectively; this is how you will fit your Nubian tunnel underneath an east-west Back Bay-Seaport subway under Marginal, and is why your Nubian tunnel needs to descend to Level 3 before hitting Marginal
  • To rise that 0.5 Levels, your tracks will need 130 feet of running space; that is easy to get on the Back Bay leg, but will require a tight turn on the Seaport leg
• Finally, when building your subway to the Seaport, I suggest provisioning a gap northeast of the Harrison/Hudson intersection (currently a parking lot); this future-proofs a Nubian-Seaport leg, either descending from a surface portal, or ascending from a subsubway that has come up under the Pike

Here is a crude diagram of what I'm describing with full build at Bay Village:

View attachment 22844

Like I said, you don't have to put the Back Bay/Seaport track directly above the others -- you could off-set it to the western side. (That decision will be partially impacted by the concourse design for the new station, which I didn't really game out the full details of.)

And here is the diagram. Hopefully self-explanatory. Darkness of the coloring corresponds to tunnel depth -- Level 1, Level 2 and so on. The effort at depicting the superimposition of the platforms is not great, but hopefully clear enough.View attachment 22848

And zoomed in. The exact measurements of the grades -- especially in the junction -- are not precise, and so the alignments may need adjusting. But there should be space for everything to fit.

View attachment 22850

Under the theory that the Nubian Branch will get built first, practically speaking this really boils down to one thing: plunk your Park-Nubian platform at Level 2.5, and if possible, dig to Level 3 before hitting Marginal.

That being said... as you can see, the envelope of the station is such that a Nubian-Park platform built at Level 1.5 could then subsequently become a lobby in a future expanded station. You could even probably carry out the work underneath while keeping the station open above, and with good staging could reduce the need for service closures. Once the lower level of the station is built you would need to lower the tunnels to the north and south, which would disrupt service but possibly could be done without excavating the entire street a second time.

And in the (highly unlikely) scenario where either a Back Bay or Seaport Branch gets built first, stacking the platforms still gives you the most flexibility for adding the second leg later on, and potentially can reduce the on-street impact of construction, due to the narrower profile.

Great work!

I wonder if there's a way to squeeze in platforms for the east-west portion of the Back Bay-Seaport subway, so that if some Riverside/Needham/Heath St trains need to be routed to Seaport to free capacity at Park and offer direct SS/Seaport connections, they can still connect to OL and Nubian trains at Bay Village/TMC. But that seems hard to do under this proposal, since the westbound track needs to start the decline from level 1 to level 2.5 immediately after passing over OL tracks.

 

[Riverside]

Great work!

I wonder if there's a way to squeeze in platforms for the east-west portion of the Back Bay-Seaport subway, so that if some Riverside/Needham/Heath St trains need to be routed to Seaport to free capacity at Park and offer direct SS/Seaport connections, they can still connect to OL and Nubian trains at Bay Village/TMC. But that seems hard to do under this proposal, since the westbound track needs to start the decline from level 1 to level 2.5 immediately after passing over OL tracks.

There’s a longer reply to be had here, but for my part I concluded about a year ago that it wasn’t worth the trouble for Back Bay-Seaport Green Line trains to stop at Bay Village. Transfer to Orange is available at Back Bay (where the transfer walk will be shorter), direct service will be available on Back Bay-Bay Village-Park trains and on Seaport-Bay Village-Park trains… and frequencies will almost certainly be high enough that it won’t make a difference. (There definitely will still be plenty of trains making those two journeys I mentioned.) Transfer to Nubian services for riders originating on the Highland Branch is a valid concern; however, there will certainly remain one-seat rides from Riverside to Park — there’s no way all Riverside trains will be diverted to the Seaport. And we should design better ways for Longwood riders to get to Nubian (ie some form of the Urban Ring), something better than riding all the way to Bay Village only to ride back out Washington.

Way upthread, you will find a proposal that does allow all trains to stop — it puts a platform on the southern leg of a triangular loop. It’s a compelling design but one downside is that it funnels every train through the same stretch of track. The design I’m presenting here would (I think) ensure that this section never is a bottleneck.

 

[Brattle Loop]

The only thing that I'd change would be the portal location (specifically the incline to street level) for the Nubian Square branch. The portal should probably be located on Washington Street immediately south of the intersection with Herald Street. This would be to ensure that the tracks are below grade through the Mass Pike Air Rights Parcels 20 & 21 in anticipation of potential future air rights development on those parcels.

That'd basically mean removing an entire block or so worth of street (even if deemed acceptable to planners, not necessarily something that would go over well in the neighborhood). I suppose it might, technically, be possible to just squeeze in a lane or two depending on how wide the incline was, but it'd still pose a big capacity squeeze.

 

[Stlin]

That'd basically mean removing an entire block or so worth of street (even if deemed acceptable to planners, not necessarily something that would go over well in the neighborhood). I suppose it might, technically, be possible to just squeeze in a lane or two depending on how wide the incline was, but it'd still pose a big capacity squeeze.

Is there any reason why there's such a hurry to climb out of the tunnel around Herald? Washington is really only under width for a portal/reservation from Herald to Berkeley. I almost feel as if extending the tunnel under Washington to portal past Berkeley would be worth it. Admittedly, there's a very large substation there with presumably lots of underground utilities in the way... Alternatively, why not portal onto overwidth Harrison, and cut across back to Washington?

 

[Equilibria]

Is there any reason why there's such a hurry to climb out of the tunnel around Herald? Washington is really only under width for a portal/reservation from Herald to Berkeley. I almost feel as if extending the tunnel under Washington to portal past Berkeley would be worth it. Admittedly, there's a very large substation there with presumably lots of underground utilities in the way... Alternatively, why not portal onto overwidth Harrison, and cut across back to Washington?

Or even (and I'm sure people know 100 reasons why you can't do this) portal on the NEC level and use the spur under Herald Street, like this:

 

[Riverside]

@themissinglink and other posts about where to put the Nubian portal -- yeah, I'm largely agnostic on that question at this point. I also am intrigued by selling Pike air rights (and certainly hope I see the Pike someday covered), though it's not clear to me that a portal where I've placed it would preclude selling the rest of the plot.

For me, the value of the alignment that I've drawn up here is that it allows you to move the portal question out-of-scope for designing the Bay Village interchange -- you can make the portal decision when designing the Washington Branch itself. Assuming this alignment holds up under scrutiny, it means that we have at least one feasible path to connect Washington St with the Tremont Subway that is only moderately roundabout and keeps the system provisioned for maximum capacity in the future. Whether you put the portal at Herald/Washington, Washington south of Herald, somewhere on Shawmut or somewhere on Harrison -- or even connecting to a subway running all the way down to Nubian -- you can hook in to this same alignment regardless.

(I do think that running all the way down to Ink Block and then doubling back is too much though. That introduces a lot of sharp turns, which will slow things down further. Plus, Ink Block itself is a three-minute walk from a surface stop at Herald/Washington, less than 800 feet on the sidewalks.)

I've also drawn up two other alignments -- one that is surface-running and uses the portal at Eliot Norton Park, and one which is more tunnel-based; the second one I am less confident about, but the first one is more feasible. I'll try to post those in the next few days.

 

[Tallguy]

Is anyone aware of any structural issues with the Pleasant St Portal?

 

[Brattle Loop]

Is anyone aware of any structural issues with the Pleasant St Portal?

If there's any information on that topic, I doubt it'd be public. The portal itself is buried but never had anything built on top of it (and while the building immediately adjacent to Elliot Norton Park looks to have been built after the portal was closed, I don't believe it ever encroached the tunnel area (let alone the portal). Part of the tunnel remains in use as an emergency exit, though the rest is disused (and supposedly contains what's left of a supply of government fallout rations). I've heard nothing to suggest that there are any structural issues with the tunnel, though I believe it would require some structural upgrades to handle the weight of LRVs as they never served there, as well a significant upgrades to roll back the clock on five-plus decades of abandonment, but that's a far cry from the kind of structural issues that would preclude rehabilitation-and-reuse or trigger concerns of potential imminent collapses. For most of the proposals in this thread, the cost of rehabbing the tunnel would pale in comparison to the cost of new tunneling.

 

[Riverside]

I mentioned that I'd been working on a surface alternative to what I presented earlier this week. My initial intention had been to create a plan for 2-phase future-proofed construction. After further examination, I think the larger pieces of that plan have fundamental flaws, particularly with respect to hooking in a branch to Nubian. So I'm going to present some more modest alignments instead (nothing too revolutionary).

Alignment A: Surface station on Shawmut:

As it happens, this is actually very similar to one of the proposals in that 1970s doc linked upthread, though I didn't know that at the time. In this design, Shawmut's three travel lanes + two parking lanes between Tremont/Oak and Marginal are mostly reclaimed for transit use. An island platform spans the length of the block, with a single southbound travel lane remaining. The Tufts Medical Center concourse is expanded west under the intersection to connect to a headhouse on the surface platform. Designed properly, the surface platform could itself be behind fare control.

South of the station, the tracks proceed through a transit-priority signalized intersection before turning southeast to a diagonal bridge across the Pike. Yes, you could also take the other two legs of the triangle, and reclaim lanes on Marginal and Washington instead. You could also then extend a branch of tracks further east to a portal at Hudson and Harrison to continue to the Seaport. This wouldn't preclude a subsequent Back Bay-South Station subway under Marginal Street, but would enable Park-Seaport service in the interim.

Pros:

• Provides a convenient transfer to the Orange Line
• Minimal tunneling (in fact, potentially zero new tunneling at all)
• Enables both Park-Nubian and Park-Seaport service

Cons:

• Adds several intersections to the Nubian Branch
• Doesn't move the ball forward at all on a Back Bay subway (although shouldn't create any obstacles either)
• Suboptimal Park-Seaport service -- if we're going to make this "round-the-bend" service pattern work, it needs to be fast, and I think surface-running with traffic intersections is going to make that challenging
• I think the grade from the portal in Eliot Norton Park to the Tremont/Oak/Shawmut intersection needs to be carefully examined. By my math, you would need 160 feet of running track to rise to surface level, and I think you can fit that in to the park's footprint... but you need to use a sharp speed-reducing curve to do it.

Alignment B: Subway station under Tremont:

Again, discussed upthread and in the 70s. Eat up the grassy shoulder and some fraction of Marginal for a portal on the block between Tremont and Shawmut. Build a center platform subway station under Tremont St, and extend the concourse from Tufts Medical Center to provide a connection to the Orange Line.

Pros:

• The grades and curves will be smoother than the alignment above
• A full subway station will create a better rider experience than a surface platform will

Cons:

• That transfer to the Orange Line is a long walk -- I believe on par with Blue to southbound-Orange at State. If we're only building for a Nubian Branch, then it's worth asking how acutely a fare-controlled transfer is required here if the Winter Street concourse is just two stops north.
• This alignment calls for much more tunneling... and still doesn't really set you up for a Boylston-Back Bay leg, a subway link to the Seaport, or a Back Bay-Seaport bypass subway
• As a result, significant rebuild (and retunneling) would be required when the time comes

Alignment C: Surface station on Shawmut, double-stack subway station under Tremont for Back Bay:

This is an intermediate build between the surface option in this post and the full-build I described earlier this week. This demonstrates how the surface station would be less disruptive to future construction of subways to Back Bay and the Seaport. What we see here could be built following the surface option, and prior to a full realignment of the Nubian Branch (requiring tunneling under the Pike). Essentially you build the Back Bay/Seaport double-stack platforms I described earlier, and leave provisions for a connection to a shallow Marginal Road subway to be built in the future. You can also take the opportunity (not shown here) to build out the rest of the subway station, even if you won't yet be running Nubian trains through it yet.

(A subtle detail worth noting: when you do hook the Nubian trains into the subway station, you will probably need to realign the tracks between the station and the old portal, as the intermediate build has them swapped compared to what I showed above. Some provisioning during initial construction of the subway could accelerate that realignment when the time comes, but it will still be disruptive.)

-----

As you can probably tell, I think Alignment B presents a high cost and a number of drawbacks. A phased construction plan would be most sensible, potentially looking like this:

1) Build Alignment A to start running LRT to Nubian as soon as possible, with minimal build
2) Expand Alignment A into Alignment C, to connect Back Bay and retire Copley Junction. (Exact timing may be dependent on when a D-E connection is built and/or when the Huntington Subway is extended.)
3) Expand the subway station to include Nubian tracks (as described earlier this week) and dig tunnel under Pike. Redirect Nubian Branch into portal just south of the NEC.

This process would also allow us to decouple the construction of the subway to South Station from all of this. A surface portal at Harrison & Hudson would enable access to South Station via the surface station during Phases 1 and 2, and then (if needed) via a roundabout path in Phase 3 (subway station, NEC portal, turn north on Washington, turn east on Marginal, enter portal). Construction of a shallow subway under Marginal to connect Back Bay to the Seaport can be therefore done at any time, since each phase includes provisions for connection.

 

[F-Line to Dudley]

Is anyone aware of any structural issues with the Pleasant St Portal?

No. It's completely intact, since the portal itself was below street-level. They just sealed the openings with bare concrete and backfilled dirt on top of it.

Inside the tunnel you'd need to reinforce the steel on the flying junction flyover ramps for modern LRV weight, obviously cleanroom all the derelict utilities, and sandblast the walls/ceiling. Other than that the tunnels are in good structural shape.