Green Line Reconfiguration

There's enough width for two, but not length. With two pocket tracks, you want a crossover between them (not on the main tracks, so that trains crossing over don't foul the opposite main track). Since the 1971-built fare lobby occupies half the length of the old platforms, there's probably not enough space for a crossover plus enough pocket length for a 225' train.
 
There's enough width for two, but not length. With two pocket tracks, you want a crossover between them (not on the main tracks, so that trains crossing over don't foul the opposite main track). Since the 1971-built fare lobby occupies half the length of the old platforms, there's probably not enough space for a crossover plus enough pocket length for a 225' train.
Just based on a rough look I think it could be doable, but it would definitely be very tight. Either way, making Gov Center into a proper terminus seems like a better use of money.
 
Either way, making Gov Center into a proper terminus seems like a better use of money.
As it happens, this is what I've been sketching over the last few days, and honestly I'm not thrilled with any of the ideas I've come up with. (For creating a modern terminus for northside trains, replacing the Brattle Loop.) So, if you have any thoughts, would love to hear them.

EDIT: In terms of modernizing the Brattle Loop to have a more reasonable turning radius, is there any reason why the utility space at the north end of the station couldn't be encroached upon to create a gentler curve? Janky diagram based on @The EGE's underground station diagram, with the revised curve in magenta:
1708045594381.png


The problem of course is that the stretch of properly tangent track gets shortened, so a full 225' train would have to open its doors on a curve. That's not as big of a problem on LRT than HRT, but it's still pretty bad.
 
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As it happens, this is what I've been sketching over the last few days, and honestly I'm not thrilled with any of the ideas I've come up with. (For creating a modern terminus for northside trains, replacing the Brattle Loop.) So, if you have any thoughts, would love to hear them.

EDIT: In terms of modernizing the Brattle Loop to have a more reasonable turning radius, is there any reason why the utility space at the north end of the station couldn't be encroached upon to create a gentler curve? Janky diagram based on @The EGE's underground station diagram, with the revised curve in magenta:
View attachment 47653

The problem of course is that the stretch of properly tangent track gets shortened, so a full 225' train would have to open its doors on a curve. That's not as big of a problem on LRT than HRT, but it's still pretty bad.
Government Center has two big advantages compared to Haymarket or Park St for example. #1 is that it's under a giant plaza, so it's not out of the question to dig up an area, build a big portion of a station, and then cover it up. #2 is that it's not boxed in like Park St or Haymarket are. The station is already quite large in terms of area, with even more space to expand further north. With that in mind, I think the dream should be more like Park St on steroids. (We can always scale down later.) Here's a quick and dirty (to-scale) mockup in Inkscape for what I'm imagining.
Screenshot 2024-02-16 at 04.08.17.png


Main features:
  1. Full quad-track between Park and GC (It seems like this should be doable with narrow track spacing, although the Park St church being where it is could make it tricky.)
  2. 6 platforms, 2 through tracks and 4 terminating platforms to actually allow for trains to wait at the terminus to regulate service when required
  3. Separated northern loop for easy reversing of regular service
  4. Additional southern loop to allow for short-turning northern trains. As this is not a regular service pattern, and whenever it is something is already disrupted, it's fine to have this as a flat junction.
  5. For cost reasons the Park St model where you can just cross the terminating tracks probably makes the most sense here.
Obviously this is not a final design, but I think it demonstrates both the concept for a modern downtown terminus and why Government Center is uniquely valuable in terms of its location.
 
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Government Center has two big advantages compared to Haymarket or Park St for example. #1 is that it's under a giant plaza, so it's not out of the question to dig up an area, build a big portion of a station, and then cover it up. #2 is that it's not boxed in like Park St or Haymarket are. The station is already quite large in terms of area, with even more space to expand further north. With that in mind, I think the dream should be more like Park St on steroids. (We can always scale down later.) Here's a quick and dirty (to-scale) mockup in Inkscape for what I'm imagining.View attachment 47660

Main features:
  1. Full quad-track between Park and GC (It seems like this should be doable with narrow track spacing, although the Park St church being where it is could make it tricky.)
  2. 6 platforms, 2 through tracks and 4 terminating platforms to actually allow for trains to wait at the terminus to regulate service when required
  3. Separated northern loop for easy reversing of regular service
  4. Additional southern loop to allow for short-turning northern trains. As this is not a regular service pattern, and whenever it is something is already disrupted, it's fine to have this as a flat junction.
  5. For cost reasons the Park St model where you can just cross the terminating tracks probably makes the most sense here.
Obviously this is not a final design, but I think it demonstrates both the concept for a modern downtown terminus and why Government Center is uniquely valuable in terms of its location.
The only reason why I'm skeptical about feasibility is the need to quad track Park-GC. I'm not on desktop now, but IIRC last time I measured it, even the street itself doesn't have any reasonable width for 4 tracks, and I highly doubt the existing tunnel is any wider (or even just as wide).
 
The only reason why I'm skeptical about feasibility is the need to quad track Park-GC. I'm not on desktop now, but IIRC last time I measured it, even the street itself doesn't have any reasonable width for 4 tracks, and I highly doubt the existing tunnel is any wider (or even just as wide).
To provide the second pair of tracks from Park to GC, I would tunnel up Park Street, then deep bore east of the State House to the proposed mega-station under City Hall plaza (see route on Google Map below). The existing 2-track tunnel under Tremont St would stay as is.

1708067821155.png
 
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The only reason why I'm skeptical about feasibility is the need to quad track Park-GC. I'm not on desktop now, but IIRC last time I measured it, even the street itself doesn't have any reasonable width for 4 tracks, and I highly doubt the existing tunnel is any wider (or even just as wide).
Given the low speed and almost entirely straight nature of the narrowest section, you could probably get away with track spacing as narrow as 3.5m. GL trains are 2.65m wide, and let's be generous and add another 2m for emergency walkways and other things. That puts us at a width of 15.15m, narrower that the narrowest ~15.5m parts Tremont St. Don't get me wrong, it would be very tight, but I think it would be wrong to just call it impossible without a deeper dive.
 
Also, if we're breaking out the deep bore machine, surely there are better uses than just reconnecting Government Center and Park.
I agree. When the Red-Blue connector is built, I don't see a need to 4-track the GL between Park station and GC.
 
#1 is that it's under a giant plaza, so it's not out of the question to dig up an area, build a big portion of a station, and then cover it up. #2 is that it's not boxed in like Park St or Haymarket are
Oh, hmm, this is an interesting point. I had been looking at reconfigurations that remained entirely within the existing envelope of the station. If we're willing to expand that envelope, then yes it becomes much more feasible to rework GC into a combination through-running/north-terminating/south-terminating transfer station.

I'm not necessarily convinced it's worth that expense though. I'm not even convinced it would be worth the (rather aggressive) internal changes I was sketching out. The need for a south-terminating loop (i.e. what the B & C use today) is much more significant than the need for a north-terminating loop. The existing Brattle Loop has a lot of spare track if you count the inner parallel tracks that branch off just south of Haymarket. It's really just the 50' curve that creates a headache, and I have to think that a modest solution for that could be found through repurposing the existing spare track.
Main features:
  1. Full quad-track between Park and GC (It seems like this should be doable with narrow track spacing, although the Park St church being where it is could make it tricky.)
  2. 6 platforms, 2 through tracks and 4 terminating platforms to actually allow for trains to wait at the terminus to regulate service when required
  3. Separated northern loop for easy reversing of regular service
  4. Additional southern loop to allow for short-turning northern trains. As this is not a regular service pattern, and whenever it is something is already disrupted, it's fine to have this as a flat junction.
  5. For cost reasons the Park St model where you can just cross the terminating tracks probably makes the most sense here.
  1. Quad Tracks: The conventional wisdom on this is that it's not feasible. Even BERy, the purveyor of Crazy Transit Pitches, did not think expanding the existing tunnel to four tracks would be viable.
  2. 6 tracks, 6 platforms: I like the thinking here. I would probably shift some pieces around.
    • I would provide access from the thru tracks to the inner platforms. Below I've drawn the crossovers so that the outer tracks can access both the middle and inner tracks, but probably middle alone would be sufficient. Provides flexibility for the thru-running services for better headway regulation
    • I like having separated platforms for southern turning services, but I think it's less necessary to have them on both the northbound and southbound sides
    • Likewise, I think you do need to keep northern turning services from blocking traffic. In your sketch, a northern turning train would have to sit on (sole) the southbound thru track, blocking traffic while it waits for a break in traffic on all other tracks in order to cross. This can be solved mostly through the added crossovers mentioned above, and in the sketch below I've created a separate holding track (with no platform) to illustrate another option
  3. Northern loop: To me, this actually points to a bigger question about turning services at GC: loops or tail tracks? Thoughts on that below
  4. Southern loop: Agree that a flat junction is probably fine here -- there will need to be a flat junction somewhere
  5. Pedestrian grade crossings: This was one of my bigger questions, more thoughts below.
Another piece to keep in mind is how to connect to the Blue Line platform. In your sketch, I think you'd need to do a down-up-down transfer tunnel similar to DTX, and I'm not sure how you'd make it accessible behind fare control without a lot of elevators. (Sketch below, purple is at/rising to Green Line level, magenta is at/descending to Blue Line level.)

1708096945972.png


Loops vs tail tracks: Loops are probably slightly faster to move through, but require more space (and/or tighter turn radius), and are harder to regulate headways with. You've addressed the latter through dual tracks leading into the loop, which is great. On the other hand, tail tracks afford more flexibility in a narrower (though longer) space. For example, the conceptual design below would allow southern terminating trains a choice of multiple yard tracks, provide a mechanism for northern trains to turn, and give thru trains the option of multiple platforms. (Many permutations of where you put crossovers.)

Also, since you don't need platforms, you can build yard tracks significantly closer to each other, giving you more turning capacity, as well as some options for storing trains downtown on a temporary basis.

1708096746006.png


Pedestrian crossings: IIRC, Park St only has one (official) grade crossing, at the northern end of the platform northbound platforms, which seems like a legacy element to me, maintained in part because of how expensive it would be to replace, and due to the relatively low speed of trains through the station, with clear(ish) sightlines for operators and pedestrians alike.

(I would also be curious for an analysis of what percentage of riders actually cross at grade there, since doors open on both sides and the main exit and Red transfer are both on the inner platform. I'm guessing crossers were mostly B -> Orange riders, and passengers using the southern exit of the station. Put another way, if operators stopped opening the right-hand doors, and fencing was put up on the outer platform's edge to direct all grade crossers to the official crossing at the north end of the platform, I wonder how much of an impact it would actually have.)

I'd argue that the Park St grade crossing is not quite "load-bearing", which is why the T is willing to live with it. Similar crossings at GC seem dicier to me, and ultimately antithetical to the goal of making the Green Line (Gold Line) more "heavy metro".

That being said, hoo boy does it make it harder to redesign the station if you can't use grade crossings.
 
I agree. When the Red-Blue connector is built, I don't see a need to 4-track the GL between Park station and GC.
Even with a relatively GL Reconfig, there could be 7 branches:
  • A to at least Brighton/Oak Sq
  • B to BC
  • C to Cleveland Circle (Or maybe Reservoir if we're feeling a bit zany)
  • D to Riverside
  • E to Needham Junction
  • 'F' to Hyde Sq (Or Arborway?)
  • 'G' to Nubian (Or beyond?)
2 branches can go north to Medford/Tufts (Or W. Medford) and Union Sq (Or Porter), and one more could go to Seaport. (I would pick E, D, and F, respectively). That leaves 4 branches that would terminate downtown plus the 2 northern branches. Any branches you turn at Park lose the connection to the BL, and any you turn at GC have to contend with the loop that crosses the northern tracks. While quad-tracking definitely isn't worth any cost, if it can be done without a deep bore I think it's strongly worth considering.

I like the thinking here. I would probably shift some pieces around.
100%, I wouldn't consider my drawing to be anywhere close to final. It's much more just a demonstration of what you can do with all the space at GC and what a modern downtown terminus could look like. As you said with the current layout BL transfers would be problematic, so some rearranging there is definitely necessary.
Similar crossings at GC seem dicier to me, and ultimately antithetical to the goal of making the Green Line (Gold Line) more "heavy metro".
That being said, hoo boy does it make it harder to redesign the station if you can't use grade crossings.
Yeah, if grade crossings are out a major rebuild is probably out too. That being said, I think an important element of this design would be how the 'light rail' branches meet the 'streetcar' branches. Running the LRT branches along the outside would mean that no grade crossings of those tracks would be necessary, and the low platforms could potentially even be raised (Or tracks lowered, same results) for high floor trains in the future. (Although that would have other challenges that we can save for another time.) TL;DR for this section: No grade-crossings of through running trains, only for the terminating streetcars.
 
I think an important element of this design would be how the 'light rail' branches meet the 'streetcar' branches. Running the LRT branches along the outside would mean that no grade crossings of those tracks would be necessary, and the low platforms could potentially even be raised (Or tracks lowered, same results) for high floor trains in the future. (Although that would have other challenges that we can save for another time.) TL;DR for this section: No grade-crossings of through running trains, only for the terminating streetcars.
To briefly reply to this point, the problem here is that restricting heavy metro to outer tracks and light metro to inners is that you lose the operational flexibility of quad-tracking, which is a pretty big hit to the cost-benefit analysis.

I’ll try to clean up my sketches to post later.
 
The original sketches are taking longer to finish than I want and I'm feeling impatient, so some thoughts from having played around with this way too much in the last few days.

Core requirements: these are the goals I would want a rebuild to achieve -- without all (or almost all) of these, I don't see how a rebuild is worth it:
  • Minimum 225' platform length (enough to berth two coupled Type 10s)
  • Minimum 10' dedicated platform depth along the entire track (see below)
  • 60' curves (based on a system where the Kenmore Loop is the ruling standard)
  • No pedestrian grade crossings
  • Minimal flat junctions
  • Full-length platforms for all services (see below)
  • Work within existing envelope of station (incl non-rev areas)
  • Turn northern services on an extraordinary basis
  • If used, tail tracks for terminating services to keep speeds high (avoid slowing down due to approaching end of track)
  • Provide enough capacity to turn a train every 3 minutes (via a loop or via a pair of reversing tracks)
I should note that basically any rebuild arrangement that avoids pedestrian crossings requires a second full headhouse to be built along Cambridge St to connect to (what is now) the southbound platform.

And while not a critical requirement necessarily, any cost-benefit analysis will be favorably impacted by increasing operational flexibility.

"Would be nice": these are worthwhile goals, but when push comes to shove, I think they can be deprioritized:
  • Easily turn northern services without disrupting thru-services (i.e. on an ordinary basis)
  • Board all/most trains in a particular direction from a single platform (see below)
  • Leaving existing stairs and elevators to the Blue Line intact
Platform length: this has been the most persistent challenge. Setting our target as tangent (straight) 10' platforms of 225' (enough to berth 2 coupled Type 10s) for each track, it's easy enough to find space for two platforms, e.g. northbound and southbound; the current station basically meets this standard already. It's also relatively doable to add a third platform somewhere into the mix, especially if you relocate the electrical substation (somehow) and expand into various non-revenue corners of the station.

But once you try for four 225' x 10' platforms, things get tight fast. So far, I haven't found an arrangement that doesn't compromise on one or more of the core requirements above. (If you reduce this requirement to "double Type 10 platforms for thru-services and single Type 10 platforms for terminating services," things do become much more tractable.)

Platform width and allocation: the width of platforms, per se, isn't the hard part; the hard part is making sure that each track has sufficient dedicated platform space. For example, a single 10' island platform for all north- and southbound services would be dangerously narrow.

The tricky part here is that there are designs that aren't cut and dry. For example, if all northbound services (thru and terminating) were distributed across three tracks with two 15' islands, perhaps that could be feasible? Likewise, a triangular platform that narrows below 10' per track at one end, but compensates with extra width at the other end... perhaps that could also be feasible?

Board all Park St trains from a single platform: definitionally, this requires all southbound trains (incl turning trains) to run on 1-2 tracks on a single platform. That precludes a dual-tails strategy for terminating trains -- at some point in the day, you will have terminating trains turning on both tracks, and thru trains will be held. So if you want all Park St trains to be accessible from a single platform, you have to use the loop

Tail track vs bay platform: In theory, you could achieve "all Park St trains from single platform" without a loop by using a bay platform, like South Station, where the terminating tracks don't continue and passengers can move between platforms by walking around the end of the tracks. As I understand it though, this requires entering trains to do so at reduced speed (to minimize risk of overruns). This will reduce capacity as trains move more slowly through the relevant crossovers etc. (We run into platform length issues here as well -- you could potentially fit a bay platform with two terminating tracks, but I think it will make things all the tighter at the junctions at the southern end of the station.)

Full-length Brattle platform: within the existing envelope, I do not think it is possible to provide a 225' Brattle platform, unless you convert one of the northern dual-track tunnels to bidirectional service (as opposed to unidirectional service today). You could, of course, do this, but you will end up "paying for it" with more flat junctions + crossings elsewhere

The value of quad-tracking: like I mentioned above, I think that simply segregating turning services onto the inner tracks and thru services to the outers dilutes the benefit of quad-tracking. Particularly with the ability to turn trains at North Station, it's likely that turning services will make up a minority of trains at GC. If used for turning "streetcar" services, I agree that the benefit increases, but I still think it kneecaps the rebuild's payoff. I would prefer thru services be able to access both sets of tracks (and ideally turning services as well)

The current loop: one other item to note here is that the current terminating loop has a flat crossing with the "inner tracks" (currently seldomly used for Brattle Loop access). If the inner tracks are reconfigured for full-time service, they will need direct access to the loop as well, which will require some reconfigurations at those junctions as well

With all that in mind, let's look at some options and the compromises they make. (The simplified diagrams below are somewhat imprecise in places and are meant to illustrate general layout. Some proposals might require slight expansions of the tunnels, shaving the walls a bit, but I suspect most of that could be avoided with more careful curve design.)
 
Option 1

Inner tracks are connected to outer tracks to the south, creating two island platforms.

1708367489470.png


The simplified diagram above shows a design that would require reworking the Blue Line stairs/elevators, but I think there is a way to do this that leaves those in place. With a second headhouse in place, transferring between light rail platforms via the Blue Line platform might be sufficient; otherwise, an underpass connection would need to be built, probably parallel to the Blue Line tunnel on the northside

Compromises: the biggest drawbacks of this design are that it does poorly on operational flexibility, and it has no way to turn northern services

Analysis: unless we're turning a lot of southern services at GC and a new underpass is not needed, this seems disruptive and modestly expensive for insufficient gain (and indeed induces a loss in ops flexibility). Particularly given Government Center's recent rebuild, this option does not seem worth it

Option 2

Same as Option 1, but with the addition of northside connections between inner and outer tracks, allowing through services access to both. With northside access to the inner loop, it would now be possible to include a Brattle Curve.

1708367509506.png


A more invasive version (illustrated above) would relocate the Brattle Curve further south to provide more platforming space for Brattle trains; a milder version could probably just leave the existing curve largely in place, though I think that would prevent even a single Type 10 consist from berthing at either platform. This design faces the same problems as Option 1 in terms of connections between the northbound and southbound platforms

Connecting the inner tracks to the loop would require some realignments; it's not clear to me whether those realignments can be done while preserving the existing switches from the outer tracks

Compromises: In general, this is a relatively balanced design. Its main drawback is that it cannot support full-length Brattle services. It also has some flexibility limitations -- thru-services can use both tracks, but terminating services cannot, creating a potential bottleneck

Analysis: this design faces some of the same obstacles as Option 1, but is able to offset them through the relatively low cost addition of a couple of crossovers and the Brattle Curve. I think it would still struggle to reach the level of being "transformative", which means it would still be hard to justify the expense and difficulty

Option 3

The same as Option 2, but with crossovers included to provide loop access from both sets of tracks. Using some simpler more abstract diagrams here, but you get the idea.

1708367531731.png


The position of the Brattle Curve is variable, with the same pros and cons discussed above

Compromises: This one does okay, all told; full-length Brattle services remain the fly in the ointment, and likewise the question of a new underpass

Analysis: This design is the first which (IMO) has the potential to be transformational. By connecting both the inner and outer tracks with both the loop and thru tracks, you finally reach an out-and-out doubling of capacity, regardless of how many services turn at Government Center. Standards ops would probably be to keep looping trains on the inner tracks as much as possible, but given this design's ability to berth two full trainsets at once in each direction (or a pair of singles), it would still be tractable to use both tracks for looping services as needed

Option 4

a.k.a. the maximally over-engineered option

Crossovers at the southern end of the station, in each of the northern tunnels, and just south of Haymarket (not pictured) enable a dizzying array of services. Of particular note is that each northern tunnel is converted to bidirectional service. This enables both southern (Green) and northern (Brown) terminating services to use either of the inner tracks as stub-ends with sufficient tail tracks to move at higher speed. The crossovers also allow thru-services to use the inner tracks... but with reversed directions.

1708367556104.png


Because of the variability of platform for services in different directions (e.g. Park St trains can board on three different tracks), a single pedestrian grade crossing is included at the southern end of the station. This crossing is gated and tied into the signal system, and fences along the inside of the inner tracks to prevent jaywalking

"Normal" operations would look more like this:

1708367581740.png


Thru services run on the outer track, terminating services in both directions use the inner tracks. Terminating Brattle services get split between the two northern tunnels.

Compromises: This one uses a pedestrian grade crossing, and lots (lots) of flat junctions. Also, while in theory terminating services won't have to reduce speed since the tracks keep going, in practice I can't imagine the T running single track bidirectional service at full speed in-and-out of a station, especially at the western part of the station where there are no sightlines. And the level of potential complexity runs the risk of washing out any ops flexibility benefits. And also you can board northbound services from three tracks on separate platforms, and the same is true for southbound

Analysis: Lol. Just lol. (I mean, not "just lol", obviously I have more to say, because of course I do.) Well, so... this design gives Brattle services full-length platforms... and it provides a method to turn Brattle trains on an ordinary basis... and it avoids revenue use of the loop in favor of tail tracks (though requiring so many flat junctions that it's surely a wash). And I guess you could actually run a service pattern similar to Option 3 on a normal basis, with flexibility to use the stub end approach when you need to run more Brattle services. But this design is almost certainly a solution in search of a problem.
 
And finally, in brief, none of the above is urgently necessary, except perhaps for doubling capacity of thru-services. The Brattle Loop is the same radius as the Kenmore Loop, so it's likely that we will be living with 50' curves for the foreseeable future. In terms of platform length, as mentioned the current platforms are long enough for supertrains. And even the Brattle platform could handle a super train as well, if the doors open on a curve. That's not ideal, but the Type 10s will be heavily articulated, so any "gaps" would be much smaller than on, for example, the Red Line. When/if the time comes for more capacity, Option 3 looks the best, but I don't think there's any huge rush.
 
The simplified diagram above shows a design that would require reworking the Blue Line stairs/elevators, but I think there is a way to do this that leaves those in place. With a second headhouse in place, transferring between light rail platforms via the Blue Line platform might be sufficient; otherwise, an underpass connection would need to be built, probably parallel to the Blue Line tunnel on the northside

Compromises: the biggest drawbacks of this design are that it does poorly on operational flexibility, and it has no way to turn northern services

Analysis: unless we're turning a lot of southern services at GC and a new underpass is not needed, this seems disruptive and modestly expensive for insufficient gain (and indeed induces a loss in ops flexibility). Particularly given Government Center's recent rebuild, this option does not seem worth it

I think this design is physically impossible, and, if not, so wildly impractical that it makes no difference. Extending the old-alignment half of the Brattle Loop to connect to the Park Street end means plowing through, at minimum, the current Blue Line elevators and the stairs from the westbound GL platform to the BL landing. And if the WB is now an island platform, the northern BL stairs/escalator would be a space concern. Doubling up the eastbound track would basically slaughter the only other existing BL stairs as well as most of the headhouse access as it currently stands. The clearances are so tight especially on the westbound side that it's extremely difficult to move anything around without hitting something vital.

Compromises: In general, this is a relatively balanced design. Its main drawback is that it cannot support full-length Brattle services. It also has some flexibility limitations -- thru-services can use both tracks, but terminating services cannot, creating a potential bottleneck

Analysis: this design faces some of the same obstacles as Option 1, but is able to offset them through the relatively low cost addition of a couple of crossovers and the Brattle Curve. I think it would still struggle to reach the level of being "transformative", which means it would still be hard to justify the expense and difficulty

Not really sure how you could possibly manage to relocate Brattle south and make it less sharp, which kinda defeats the purpose. (Then again, I'm pretty bad at any math involving circles.)

And finally, in brief, none of the above is urgently necessary, except perhaps for doubling capacity of thru-services. The Brattle Loop is the same radius as the Kenmore Loop, so it's likely that we will be living with 50' curves for the foreseeable future. In terms of platform length, as mentioned the current platforms are long enough for supertrains. And even the Brattle platform could handle a super train as well, if the doors open on a curve. That's not ideal, but the Type 10s will be heavily articulated, so any "gaps" would be much smaller than on, for example, the Red Line. When/if the time comes for more capacity, Option 3 looks the best, but I don't think there's any huge rush.

The Brattle Loop platform's a logistical nightmare, I would hope the T10s would help to some extent. The existing platform can't quite fit a two-car train of T7/8s, though I'm not entirely certain if it has to do with the doors. (They definitely cut back the raised platform from where it was when GC reopened, so it wouldn't surprise me if the folding doors would hit the edge.) Even using the old section has problems, because there's a huge column on the westbound side that's definitely structural, meaning a big stretch of the westbound platform can't be used by Brattle trains. The side platform is there, of course, but then you're back to pedestrians crossing the ROW. (Though when GC was built out of Scollay, the Brattle Loop was paved, so...)

Just realized I never actually posted the "current state" diagram, drawn from the renovation project presentation. A few details have been simplified (don't rely too much on the doors into the rooms on the non-rev side of the Brattle Loop, or the particulars of the electrical substation, for example).

Also missing quite a few support columns, some of them a century-plus old, though to be fair the original also seems to have missed a fair few.

Completely random note, but why is there that notch in the wall behind the Brattle curve? What was it for, if anything?
 
The simplified diagram above shows a design that would require reworking the Blue Line stairs/elevators, but I think there is a way to do this that leaves those in place. With a second headhouse in place, transferring between light rail platforms via the Blue Line platform might be sufficient; otherwise, an underpass connection would need to be built, probably parallel to the Blue Line tunnel on the northside

Compromises: the biggest drawbacks of this design are that it does poorly on operational flexibility, and it has no way to turn northern services

Analysis: unless we're turning a lot of southern services at GC and a new underpass is not needed, this seems disruptive and modestly expensive for insufficient gain (and indeed induces a loss in ops flexibility). Particularly given Government Center's recent rebuild, this option does not seem worth it
I think this design is physically impossible, and, if not, so wildly impractical that it makes no difference. Extending the old-alignment half of the Brattle Loop to connect to the Park Street end means plowing through, at minimum, the current Blue Line elevators and the stairs from the westbound GL platform to the BL landing. And if the WB is now an island platform, the northern BL stairs/escalator would be a space concern. Doubling up the eastbound track would basically slaughter the only other existing BL stairs as well as most of the headhouse access as it currently stands. The clearances are so tight especially on the westbound side that it's extremely difficult to move anything around without hitting something vital.
Well, I did indeed manage to find a design that preserves the Blue Line elevators... and it is ugggggly.

1708434623369.png

The part that becomes near-physically impossible, as you rightfully point out, is between the eastern Blue Line stairs:

1708434860837.png

To keep those in place, you need to thread both of the inner tracks between the stairs, and leave enough space for the western ("streetside") track to curve around the elevators. Which then means that the 225' platform can't start until the track clears the elevators... which then means you completely blow out the eastern wall of the station to curve around into the loop.

(If the streetside stairs can be rotated and slightly relocated, it might be possible to thread both inner tracks through the gap, but you'd still run into the other problems.)
Doubling up the eastbound track would basically slaughter the only other existing BL stairs as well as most of the headhouse access as it currently stands. The clearances are so tight especially on the westbound side that it's extremely difficult to move anything around without hitting something vital.
(I assume by "eastbound", you mean "northbound" right? The east-west model makes a lot less sense to me now that GLX is open.) I was a little less worried about redoing the stairs and headhouse etc on the northbound platform -- my main priority was simply to work within the existing footprint of the station. Any version of this rebuild will not be cheap, at all. If it becomes a requirement to completely leave the headhouse and existing stairs untouched, then yes, this whole idea is DOA (though, see caveat below).

My vague recollection is that F-Line suggested some sort of in-station overpass to connect the two island LRT platforms, but I can't remember how, and can't see how that would work.

The caveat is that, again without a huge amount of confidence, I think that if the platform requirement for the inner tracks is reduced (i.e. to berth only one Type 10), then you get more room to maneuver and so perhaps could swing this.
Not really sure how you could possibly manage to relocate Brattle south and make it less sharp, which kinda defeats the purpose. (Then again, I'm pretty bad at any math involving circles.)
I'd have to go back and check my notes, but I think I wasn't worrying about increasing Brattle's turn radius, since Kenmore's radius is the same and is here to stay. That being said, this below is how the radius could be increased within the existing footprint:

1708437580654.png


Black is the current, pink is an unspecifiedly larger radius.
Even using the old section has problems, because there's a huge column on the westbound side that's definitely structural, meaning a big stretch of the westbound platform can't be used by Brattle trains. The side platform is there, of course, but then you're back to pedestrians crossing the ROW. (Though when GC was built out of Scollay, the Brattle Loop was paved, so...)
I'm slightly less concerned about passengers crossing the Brattle ROW. That seems vaguely comparable to the Park St grade crossing, in that volumes would not be super high and service not maximally frequent. But yes, yet another problem.
Also missing quite a few support columns, some of them a century-plus old, though to be fair the original also seems to have missed a fair few.
Yeah, as mentioned above I was assuming that this would be a Big Project, meaning that a lot of structural work within the station would be in scope.

Of course at some point, you need to ask whether it would be cheaper to build within the existing footprint, or better just to do a full rebuild like @TheRatmeister proposed.
 
Just to explore every single nook and cranny of this wholly unnecessary exercise:

1708450776026.png

Consolidate all services on to a single plazaside island platform. Convert streetside platforms into a small yard. Terminating services depart GC northbound platform and loop around and rejoin the southbound track just south of the platform to head to Park St, or go to yard for OOS layover. Platform extended north into substation area.

Headhouse and its elevators + stairs are untouched. Streetside Blue Line stairs closed. Blue Line elevators relocated into the vicinity of the current southern stairs' southern stairs. (That sounds more confusing than it is.) The southern stairs' southern stairs are filled in to provide the landing for the elevators.

What does this buy you that today's station doesn't? Not much. The main thing is one, maybe two storage tracks for terminating services. Plus I guess terminating services get a bit more dedicated real estate in terms of trackage, so you potentially get a bit more capacity as you are effectively relocating the merge point from north of GC to just south of it.

But... really, this is not worth it.
 
Just to explore every single nook and cranny of this wholly unnecessary exercise:



But... really, this is not worth it.
I agree. Transit funding is incredibly scarce, and there are several supremely important projects that need to happen, so this would be a low priority for sure.
 

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