I-90 Interchange Improvement Project & West Station | Allston

F-Line to Dudley said:
  • 1 center express track. No crossing conflicts between locals and expresses.
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This brings a two-part question to mind:

1. Why is it sometimes ok to have a center (bi-directional) express track, rather than one express track for each direction? Milford Station is set up the same way, and being in Metro-North territory on the Northeast Corridor, it no doubt handles much more traffic than West Station would. Is it mostly a matter of having universal crossovers right near the station?

2. Since West Station doesn’t seem to have any ROW width constraints that would make a four-track station design wildly more expensive than a three-track design, why choose the bi-directional express track? Why subject yourself to the inbound/outbound schedule dependencies if you don’t have to?
 
riptide said:
This brings a two-part question to mind:

1. Why is it sometimes ok to have a center (bi-directional) express track, rather than one express track for each direction? Milford Station is set up the same way, and being in Metro-North territory on the Northeast Corridor, it no doubt handles much more traffic than West Station would. Is it mostly a matter of having universal crossovers right near the station?

2. Since West Station doesn’t seem to have any ROW width constraints that would make a four-track station design wildly more expensive than a three-track design, why choose the bi-directional express track? Why subject yourself to the inbound/outbound schedule dependencies if you don’t have to?
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There's not really going to be any inbound/outbound schedule dependencies on an express track that short. Minimum thru-Worcester headways are going to be :30 bi-directional, meaning there will only be an expressing train passing one direction or another every 15 minutes. Amtrak's tippy-top schedules are 8-10 round-trips per day, which interspersed with the thru-Worcesters is likewise too little traffic to saturate a passer that short. The track will get cleared by any movement in less than a minute, so even a sub-5 minute meet wouldn't have any conflicts. The universal crossovers do keep everything nice and elegant, and having the passer in the center keeps the conflicting movements to an absolute minimum.

The reasons for choosing a bi-directional express track instead of two unidirectional express tracks are:
  • It's excessive infrastructure for the traffic levels. It's one thing if you're the New Haven Line and you've got 4 contiguous tracks the whole way to mix frequent local and express patterns...eliminates nearly all crossover moves and keeps things running fast from the lack of little performance hits while crossing over. Worcester express and Amtrak service will already need to play a couple sets of crossover games on the Regional Rail service layer cake to hop over the Newton stations on the unexpandable 2-track ROW, so an extra set of crossover moves in Allston is not meaningfully going to slow down inside-128 express service more than it already (not much) is.
  • It simplifies the complicated interlockings. The crossovers will already be spanning 4 tracks on the east end for allowing universal movements on/off the Grand Junction and to/from any West platform berth, and will be merging 4 tracks into 3 to the west and shifting the express traffic from the center to the outlying Grand Junction track for passing Boston Landing station. Adding additional complexity to both ends with an extra express track will slow down the trip a little bit further, create lots more maintenance complexity, and drive up cost while ^above^ the traffic levels don't quite merit it. You could *possibly* consider it if the Grand Junction were LRT from Day 1 since that would eliminate half the complexity of the interlockings (even then it would be excessive-to-task), but not while it's already one of the most complex mainline interlockings on the system.
  • The extra space is put to better use provisioning for the future LRT island platform and extra LRT track berth, correcting the lack of provisioning in MassDOT's hot-mess plan.
  • The Pike is tantalizingly close to being arrow-straight with the compacted works even with the LRT provision left, so it's better all-around to keep the bulb-out to a minimum especially when it doesn't affect rail capacity in any tangible way.
 
The EGE said:
View attachment 60991

View attachment 60992

You physically cannot have a switch next to a high-level platform. Any train making the diverging move (using the orange track) will hit the platform.
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A correction: it is physically possible - here's a (rare) example at New York Penn:
1742063491650.jpeg


But you have to move the platform edge back, and thus deal with a larger gap, to make it work. Maybe worthwhile in rare cases like Penn Station where you need the flexibility, but absolutely no justification here.
 
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