Option 1
Inner tracks are connected to outer tracks to the south, creating two island platforms.
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.
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.
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.
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:
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.
