Green Line Reconfiguration

[Riverside]

@Teban54, thank you (as ever) for the detailed and thoughtful reply!

I think I agree with you about the theoretical dimensions of limitations here. Obviously the particular practical effects of each of those dimensions can be discussed and examined in much greater depth, but I think I agree about the framework and which factors influence.

Like I said, I think my personal preference is for a setup that ends up rendering most of the concerns discussed here moot (admittedly in part by settling for something less than the full 80 tph capacity -- 16 tph on Huntington-GC, 16 tph on Nubian-GC, ~32 tph on Kenmore-Seaport, and x tph using Park Street Inner for extra service as needed).

Two relatively small notes:

Crossovers (switches): yeah, I noticed the same southbound crossover situation you did; to be honest, I assumed (perhaps naively) that it would be trivial to add a new diamond along there. If we're talking about building a new subway along Marginal Road (to say nothing of the more elaborate suggestions in this thread), my hope is that it would be easier to add a new crossover in an existing tunnel. But it's totally a fair point that I don't know any of the potential complexities around that.

Crossovers (trains):

Assume that you have 32 TPH from Huntington and 16 TPH from Nubian. Under your proposal, all trains need to switch north of Boylston (Huntington trains from Boylston Inner to Park Outer, Nubian trains from Boylston Outer to Park Inner). All trains cross each other's paths, so you'd then need to slot every Nubian train in-between adjacent Huntington trains -- but if Huntington is operating at full capacity, there won't be any such gaps.

Strictly speaking, I don't think this is entirely the case. 32 TPH from Huntington means a departure every 1m52s. I took a quick look at a couple of crossovers out there: at Union Square, the crossover is 180'; in Santa Monica, their E has a 210' crossover. At 10 mph, 210' could be cleared in 15 seconds. Even at worst case, at 5 mph it could be cleared in 30 seconds. So even if you had a Huntington and Nubian train departing Boylston at the exact same time, the worst case is that the Nubian train has to hold for 30 seconds -- but hopefully more like 15 seconds, which seems manageable (particularly if Nubian trains are "only" running every 3m45s on average).

All of which is to say, "full capacity" does not necessarily entail the crossover being blocked at all times.

(I am mindful that TheRatmeister made a similar argument to me a couple of months ago and I voiced skepticism there. So perhaps I need to go back and reexamine my assumptions there -- or perhaps my argument here isn't particularly strong.)

 

[Teban54]

Strictly speaking, I don't think this is entirely the case. 32 TPH from Huntington means a departure every 1m52s. I took a quick look at a couple of crossovers out there: at Union Square, the crossover is 180'; in Santa Monica, their E has a 210' crossover. At 10 mph, 210' could be cleared in 15 seconds. Even at worst case, at 5 mph it could be cleared in 30 seconds. So even if you had a Huntington and Nubian train departing Boylston at the exact same time, the worst case is that the Nubian train has to hold for 30 seconds -- but hopefully more like 15 seconds, which seems manageable (particularly if Nubian trains are "only" running every 3m45s on average).

All of which is to say, "full capacity" does not necessarily entail the crossover being blocked at all times.

The interesting question is, if the only concern is "how long it takes to clear a switch"... Then why can't the capacity limit of a track be higher than 32 TPH in the first place?

In my amateur view, a capacity limit comes from:

1. Turnback capacity at each individual terminal
   • At each terminal, the train not only needs to go through the switch, but also reverse direction, load and unload, etc. The bottleneck is often largely due to how long the train occupies the terminal track (beyond the last switches), not the switch itself.
2. Dwell time within each station; signals between adjacent trains; as well as other typical variations in runtime, even when just running along a straight track
   • Headway = Time between (the front of the first train entering the station) and (the front of the second train entering the station)
   • Between these two time stamps, a lot of things need to happen:
     • a) The front of the train needs to reach the end of the platform and stop
     • b) Loading and unloading passengers
     • c) The entire train then needs to leave the platform
     • d) The second train then needs to traverse through the gap between the two trains -- as directed by signals -- that's meant to prevent collisions
   • Between Feb and Apr 2025, the median dwell time at Park St is 74s southbound and 73s northbound; Government Center 56s southbound; Copley 53s northbound
     • I'm not even sure how many of (a,b,c,d) these account for, so the actual minimum gaps between two trains may be even longer
   • Based on my understanding, this is why people generally assume that most heavy rail lines have a minimum headway of 2 minutes. A few systems can achieve 90 seconds with CBTC, but they're exceedingly rare.

The thing is, I believe #2 is a more crucial factor than #1. If #1 were the bottleneck, you would be able to unlock higher headways by building multiple terminals or operating a ring route -- which is exactly what the ultra-frequent heavy rail lines do.

Back to the specific question of X-merges. In terms of crossover conflicts, you can avoid (b), but (a,c,d) still apply, just that "platform" is replaced with "crossover". In other words, after the preceding Huntington train passes through the crossover, and then after (d), you need a Nubian train to complete (a,c) at the crossover (aka go through the switch) while the next Huntington train performs (a,b) at the platform (aka enter Boylston and load passengers).

The only way this can work out is... If (b) is slow enough but (a,c) are quick enough. Or in essence, dwell time is primarily limited by passenger flow rather than travel. But due to the following bullet point, I doubt that's the case:

• A factor that you might not have factored in: Train length also counts towards the "crossover length". A crossover is occupied from the moment the front of the train enters it to the moment the rear of the train leaves it. A two-car Type 10 would be about 227 ft long, which should be added to the effective length.

I'm willing to be convinced otherwise, but so far I'm not: I believe that any capabilities that would allow Nubian trains to "nicely" slot between Huntington trains could have also been used to add more capacity on any single track in the first place, thus violating the 32 TPH assumption.

 

[Riverside]

I'm willing to be convinced otherwise, but so far I'm not: I believe that any capabilities that would allow Nubian trains to "nicely" slot between Huntington trains could have also been used to add more capacity on any single track in the first place, thus violating the 32 TPH assumption.

Yeah this is fair, I agree.

 

[TheRatmeister]

Strictly speaking, I don't think this is entirely the case. 32 TPH from Huntington means a departure every 1m52s. I took a quick look at a couple of crossovers out there: at Union Square, the crossover is 180'; in Santa Monica, their E has a 210' crossover. At 10 mph, 210' could be cleared in 15 seconds. Even at worst case, at 5 mph it could be cleared in 30 seconds. So even if you had a Huntington and Nubian train departing Boylston at the exact same time, the worst case is that the Nubian train has to hold for 30 seconds -- but hopefully more like 15 seconds, which seems manageable (particularly if Nubian trains are "only" running every 3m45s on average).

All of which is to say, "full capacity" does not necessarily entail the crossover being blocked at all times.

(I am mindful that TheRatmeister made a similar argument to me a couple of months ago and I voiced skepticism there. So perhaps I need to go back and reexamine my assumptions there -- or perhaps my argument here isn't particularly strong.)

I believe I was talking about merging street service from Hyde Square into the Huntington Ave subway there? Correct me if I'm wrong.

There's a pretty big gap in the service levels between the two cases, and also a big difference in the actual switching going on. If an underground Mission Park station was built like Kenmore, Hyde Square trains could use the middle (or side, doesn't matter) platforms and then merge into the Huntington Ave subway. No crossing paths, just wait for a gap and then go. If they need to hold for 30-60s to keep service running smooth, so be it, that's just what happens to surface light rail sometimes. But the frequency assumption there is different, the 4-8 Hyde Square trains per hour need to slot into the gap left by the 18-24 Huntington trains per hour. At worst that's a gap of 2.5 minutes for a train to go, very doable.

Another possible problem with a crossover service like this is that it has a major single point of failure. If one of those switches, a moving part which will inevitably break at some point, gets stuck one way or the other (or worse, in the middle), now service is completely borked on at least the Huntington branch until someone gets out to fix it. It's a similar 'risk' to that introduced by switches at terminals to turn trains. If one gets stuck there you lose a bunch of capacity from single-tracking, if one gets stuck here you lose a bunch of capacity from needing to send 32 Huntington TPH through the ~20TPH Park St Loop. If it's not necessary I see no reason to introduce that risk, and quite simply I don't think it's necessary here. I think 32 TPH for Nubian+Huntington is probably enough, and if we want more then the Charles St Subway is there as an option to complement 'SL Phase III Revised.' That also leads into why I like the Essex St route. Despite it's many complexities it balances distance, transfers, and the ability for a future Nubian/Huntington Split better than any alternative I've really seen so far.

Spoiler: Service Breakdown for 32TPH Combined

Branch	TPH
Nubian	10/12
Riverside	7
Needham Jct.	7
Hyde Sq	4/6 (To be suplemented with additional trains to Kenmore)