I don’t have much to add to Yonah Freemark’s post about Boston’s proposed Fairmount Line infill; as Yonah correctly notes, this is a good idea in principle, but in practice it also requires operational integration, especially unified fares. The current federal aid system gives agencies a large incentive to install concrete, some incentive to install electronics, and none to improve organization.
What I want to discuss in this post is the myth that Boston’s South Station has capacity problems, a myth almost as pernicious as the same myth about New York’s Penn Station. While South Station can’t immediately solve all of its capacity problems with through-running (P.S. note the cost estimates for 2.4 km of tunnel in Boston are $3-9 billion), it still has enough tracks for service increase. Thus the 20-minute frequency limit mentioned in the comments to Yonah’s post is not as binding as the MBTA may think.
South Station has 13 tracks. These naturally separate into a group of 4-5 to the east and a group of 8-9 to the west. The eastern tracks are fed by a four-track bridge serving the Fairmount Line, the Old Colony Lines, and the Greenbush Line. The western tracks curve 90 degrees (with radius, I believe, 250 meters, limiting approach speeds) west and become a four-track line reaching Back Bay, and fanning out to the Worcester, Providence, Needham, and Franklin Lines; the Providence Line also hosts Northeast Corridor intercity trains, while the Worcester Line hosts a single daily Amtrak train.
For all intents and purposes, the two sets of tracks should be treated separately, for the following reasons. First, any train, any track is good to have as a contingency, but should not be done regularly, in order to make service as predictable as practical. Second and more importantly, the capacity of a terminal is far higher when the trains are completely interchangeable, as they are to the east. If slight schedule irregularities create conflicting terminal moves, the run can be done from any track.
In the simplest case, that of a two-track line hitting a two-track terminal with (short) tail tracks, the turning capacity can approach 30 trains per hour, the same as that of a running line; see for example the schedule, satellite view, and station map of the Chuo Rapid Line. This is uncommon, but many other commuter lines in Japan turn 12-15 tph on two tracks.
The four-track eastern segment of South Station can be split without revenue conflicts into two western tracks serving Fairmount and two serving the other lines, and such capacities become realistic. Since total peak traffic on the Old Colony and Greenbush Lines is currently 6 tph, and total peak traffic on the Fairmount Line is 2 tph (should be 6 tph for good urban service), capacity there is a non-issue. Although there are no tail tracks at South Station, all platform tracks except the easternmost are long enough that they could attach to platforms a few tens of meters longer than an eight-car commuter train, which with modern rolling stock should suffice.
The western tracks pose a bigger problem, for two reasons. First, the trains are not perfectly interchangeable, and do not separate neatly into two two-track lines running alongside each other. Second, Amtrak should be planning on 400-meter trains, and although the platforms could be lengthened to accommodate them, tail tracks become impossible, forcing even slower approach speeds than required by the curve.
Regardless, South Station has enough capacity even for trains serving Back Bay. With completely non-interchangeable intercity trains and dwells that are long by regional rail standards, the Tohoku Shinkansen turns a peak of 14 tph using four station tracks at Tokyo. While the Tohoku Shinkansen does not have the sharp turn of South Station, the MBTA can turn trains faster (trains already turn in about 5 minutes at the outbound terminals), and all services but one use the same equipment. So the capacity for South Station West is at a minimum 28 tph; current peak traffic excluding Amtrak is 12 tph.
It goes without saying that the operating assumption I’m using is that service is run well, better than is currently possible under the FRA-regulated regime. Among the FRA’s sins is brake tests at every terminal, forcing longer dwell times than are routine in Japan, France, and other countries with a much safer rail record than the US, to say nothing of American rapid transit (which outside Washington D.C. is very safe). While all of the above examples of high turn capacity use EMUs with high acceleration and deceleration, the separation between maximum capacity and current MBTA traffic is high enough that large service increases are possible without either more concrete or more electronics; with better electronics, even more increases are feasible.
I am going to return to this issue, specifically the Providence Line, because one way to save some money on Northeast Corridor improvements is to speed up the Providence Line, using existing electrification and new rolling stock; this would permit the line to remain two-tracked with one mid-line four-track passing segment around Sharon, obviating the need for Amtrak’s proposed third track, even with large increases in ridership.