Re: Driven By Customer 'Service' Parte Dos
Prior to the experiment with operating some three-car trains in Fall 2010, the Green Line peak headways for a portion of both the AM and PM peaks was every 5 minutes on the D and E, slightly shorter than every 5 minutes on the B (a 5/5/4 pattern) and slightly longer than every 5 on the C (5/6/6). If you stood on the platform at Copley outbound from 5 to 5:30 PM and counted the number of trains you saw each fay for that half-hour over a week, the average for the 30-minute observed period would have come out to about 24 trains. Since the scheduling of some three-car trains, there has been some reduction in that amount.
From a service perspective, reliably hitting 6-7 minute intervals on the branches with longer trains is much better than more trains, shorter headways and more bunching. Train throughput is only one part of the equation; if they're supposed to be hitting 5-minute intervals but perpetually missing them, it doesn't do anyone good since trains are expressed, short-turned or go out well above/below capacity.
There comes a point in a slow speed (because of other constraints), high-frequency operation when a CBTC system would not allow as close a spacing as line of sight. An extreme hypothetical example: CBTC would not have allowed for an operation like this had it existed 115 years ago:
http://www.flickr.com/photos/boston_public_library/5143673992/in/set-72157625136193751/
But I think this is losing sight of a main issue: the Green Line was never meant to operate the way it currently does with the technology it has. We need more three-car trains at regular intervals, not 48+ trains with the same messy, manual and perpetually reactionary scheduling.
Although the MBTA is scheduled (or was scheduled through Fall 2010) for 48 trains per hour during the busiest parts of the peak, the system has to be able to accommodate an even higher number when recovering from a delay. I have heard that in practice vs. theory, MUNI has found it difficult to schedule beyond 35 trains or so per hour on the 5 lines it runs (the K and T operate as a through service, in part because of capacity constaints). Note that MUNI metro is 100% high platform in the signalized tunnel (thus shorter dwell times), has longer station spacing in the subway vs. the Green Line, has higher operating speeds on relatively tangent track, and doesn’t have conflicting moves at flat junctions within the subway. All things that could dilute performance if an identical CBTC system to MUNI’s was installed on the Green Line.
The number of trains is almost a secondary concern to keeping consistent headways, something which the Green Line is abysmally terrible at. The reason why it is so overcrowded at times and normal at others is because they'll throw 5 trains down the line in rapid succession and have a gap of 6-10 minutes where no trains show up - during rush hour - meaning most trains are already packed by Park Street. At least with a CBTC system trains would be able to automatically adjust to congestion by moving with the blocks at a reduced speed (thus not using as much energy stopping and starting) and smoothing out the variances in headways much more easily.
I don’t think they have encountered too many problems operating them on the B and the D, but they do have concerns that too many 3-car trains in any one power section could blow a sub-station. That would require upgrades to the electrical system if they were to consistently operate more 3-car trains. The Heath St. loop can only hold a two car train, that would be a problem for operating them all the way on the E line. For the C line, the North Station turn-back would need the interlocking reconfigured so that a 3-car train changing ends would clear the interlocking. I also believe some of the platforms on Beacon St. might have to be extended to hold all of the doors on a three-car train.
A potential future operating scenario, with enough equipment and enough power avialable, would be to operate 3-car trains every five minutes on the B and the D and two-car trains every five on the C and E. I think the computer demand models for GLX service patterns make assumptions about three-car trains on the D Line.
I wasn't referring to them having problems operating 3-car trains, but rather operating
all 3-car trains which, as you point out, would be impossible. You are correct that there are worries about electrical requirements on all lines and station lengths along the C and E are not adequate. Also, given that they've shifted away from 5-minute headways with just 32 daily 3-car trains, I'd say it is an impossibility for them to run 5-minute headways on an all 3-car operation anyway.
Also as we're talking about 3-car trains, I realized I haven't seen a 3-car on the E branch in quite some time - seems like they nixed the Brigham Circle short turns?
Everyone who could leave and get a better job in the private sector did.