A few thoughts here.Shower thought: In a world where the "parallel" model is implemented and all Kenmore trains terminate at Park St, how feasible would it be to build a pair of crossovers south of Park St and convert it into a double stub-end terminal? Like this:
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While I believe the loop is still more efficient as a terminal, this adds flexibility to hold trains in the terminal for headway control. Someone with more knowledge in operations can correct me if I'm wrong.
First, my vague understanding is that these days the gold standard for terminals is a crossover leading into a pair of tracks (ideally with a center platform) followed by another crossover and then a set of tail tracks long enough for at least one trainset each. This provides storage space for sets that either need to be temporarily taken out of service, or for staging to prepare for peak periods. This also provides a high level of flexibility -- dispatchers can send terminating trains into either platform track and subsequently either back into service through a reverse move, or out of service to either of the tail tracks. (And the center platform means waiting passengers can easily access departing trains from either track.)
It is true that it takes a little longer to turn the train because the driver needs to switch ends, but having a brief break from revenue service isn't always a bad thing.
A loop, on the other hand, basically means that the train stays in revenue operation for the entirety of the turn, effectively doubling the length of the line. No opportunity to hold trains, no opportunity to let trains leapfrog each other... if three B's arrive into Park Street back to back, then you're gonna have three B's departing back to back as well.
So, yes, I would feel better about the inner tracks at Park Street if they had a crossover south of the station and could be more like a stub-end terminal. That would allow at least some flexibility in holding and reordering trains. The challenge I see is that the inner tracks lack a central platform; it'll be pretty obnoxious for a commuter to be on the western platform and discover that their train is actually departing from the eastern. I was looking at old plans the other night to get a sense of whether tracks could be rearranged to create a center platform, and I wasn't super optimistic about doing so unless the footprint of the station were expanded.
Oh, I agree -- it's one of those "extraordinary claims require extraordinary evidence" situations, where you'd need to make a really convincing case that it would be worthwhile.I actually really like the locations of Arlington and Copley stations, having used Arlington extensively to get to/from Public Garden myself. An argument can also be made that Copley station serves the Back Bay area better than the namesake station on OL (and the Huntington subway). For these reasons, I raise eyebrows at any proposal that reduces Copley and Arlington to a shuttle to Park St, but it may be just me.
I took a look at this earlier today. My estimate is that a 4-track ROW (assuming side-by-side tracks) would need to be 50 feet wide.Another potential concern for a 4-track Back Bay subway is the width under Marginal St and the complexity of Bay Village. However, I think there's an alternative: A Stuart St subway, which was mentioned by Van in some of the discussions you linked earlier, in addition to a Marginal St subway. Not sure if it's a good idea for both subways to co-exist given complications with OL transfers and alignment near Pleasant St portal, but I suppose that in a real-world proposal, both subways will be studied.
- Huntington - Dartmouth: should be fine
- Dartmouth - Clarendon: this is an area of concern since the width would need to increase significantly to fit platforms. Possibly this could be ameliorated by off-set platforms, such that the width is never greater than 4 tracks + 1 platform
- Clarendon - Berkeley: would need to build under at least one building, but the 2-track alt needs to do the same
- Berkeley - Arlington: looks like there is amply more than 50 feet between the edges of Cortes St and the Mass Pike; you'll need to fuss with the on-ramp, but again that's the same as the 2-track alt
- Arlington - Tremont: here we have some constraint; however, we could probably stack our tunnels here, 2 over 2
I think Stuart Street is a useful alternative to keep under consideration, but I suspect it will lose points when it comes time to design the interface with the old Tremont Street Subway; you'd be unable to leverage the flying interchange available at Pleasant Street, which then opens up a can of worms.
I think it depends on whether you are looking at flat junctions or flying junctions. If you're talking flat junctions, then it's easy-peasy of course. If we're talking about flying junctions, I think it'll be very hard. A flying junction requires both depth and width, neither of which are in large supply in that area.Yet another shower thought: Does there exist a design around Copley that enables three - or even four - of the routings below?
- Kenmore - Arlington
- Kenmore - Back Bay
- Huntington - Arlington
- Huntington - Back Bay
I tried sketching this out (reminded me of the headache it was to sketch out the Bay Village junction), but eventually gave up. Maybe there's something possible? (Like maybe you could do something with flat splits/crossovers between tracks going in the same direction, with grade separation between eastbound and westbound traffic?)