Last year (oh, wait, in 2012—gosh) I wrote about the sorry state of affairs of the MBTA's commuter rail system. Outside of rush hour, trains travel infrequently and on seemingly random schedules. Even in closer-in, denser areas, service is provided only every couple of hours. (Oh, yeah, and service is slow, too.) There is probably demand for better service but little supply, and it's partially a cost issue: running full, 1000-passenger commuter trains (with poor acceleration) is a losing proposition when few seats are filled.
Enter the DMU. The diesel multiple unit, while sparsely used in the US, is a frequent sight abroad, providing service at times when a full trainset is not needed. They leverage the direct connections between town centers that exist along rail lines, allowing fast service to the city center without rolling empty cars through the midday and evening. The plan imagines several lines utilizing DMUs on rail segments inside 128—most of which do not have paralleling subway service:
If implemented, this would go a long way towards improving service on several underserved corridors. The Fairmount Line currently is served infreqently by full-length commuter trains, a complete mismatch of service. Lower capacity and better acceleration would provide a much better benefit there. The Worcester Line—which will soon have added service—provides very little service in Newton, the most densely-populated portion of the line (it is served by express buses, which crawl through traffic on the Turnpike). Better service to Lynn will provide some of what a far-costlier rapid transit connection would—faster and more frequent service to Boston, and service to Woburn will provide better connectivity north of Boston, as well as further reducing the run times for trains to Lowell, currently one of the faster trips in the system, and one which, if the state of New Hampshire pays, could be extended northwards to the granite state to provide an alternative for the traffic-choked I-93.
The most interesting piece here is the inclusion of the Grand Junction through Cambridge. This page would argue for a complete rebuild of this line, with grade separation and electrification, and its operation as a crosstown link between North Station, Cambridge and Allston, currently a serpentine trip by car, much less by transit. It also would provide better access to the fast-growing Cambridge area which is currently poorly-served by highways and at the whim of the over-capacity Red Line. Currently, commuting from west of Boston to Cambridge is a bit of a black hole; there is no good connection between the Worcester Line and Cambridge without going all the way to South Station and backtracking across the Red Line. The Grand Junction is an underutilized piece of infrastructure which could be put to very good use.
What really matters is how the MBTA decides to implement DMU service. (That is, if it can be funded and overcome local opposition; double-dipping Cambridge City Councilman and State Rep Tim Toomey helped to quash a plan to route some commuter service over the Grand Junction in 2010. Needless to say, he did not receive my vote in recent council elections). If it uses it as a cost-saving measure to run the same level of service with less equipment, it will simply maintain the status quo. For instance: there are only eight commuter rail trains from the stations in Newton to Boston daily. And the Fairmount Line has 60 minute headways with no evening or weekend service. Running the same level of service with different equipment would be a wasted opportunity (if a minor cost savings).
If the state provides faster and more frequent service to these lines, say, with 15 or 20 minute headways all day, it will come much closer to providing a transit level of service, and actually providing service to these communities. With new stations at Yawkey, near New Balance and the intermediate "West Station" the Worcester Line would provide better connections through an underserved portion of Boston and Newton (with frequent, timed shuttle service across the Grand Junction a dramatic bonus). And the Fairmount Line, which currently provides piss-poor service to one of the states most disadvantaged communities, could prove an economic lifeline, if only the trains ran more than hourly.
And if DMUs are successful, they could be implemented in other parts of the system. Why run a full trainset at off-peak hours from Newburyport to Boston when a transfer at Beverly would provide the same level of service? Perhaps instead of the hourly locomotive-hauled train from Boston to Lowell, half-hourly DMUs would double the service at a minimal additional cost. Perhaps DMUs could provide service further west of Worcester to Springfield and Amherst. Hopefully, the state sees DMUs as a tool, not as a cost-cutting measure. If they implement this service better than they photoshopped the map, they could dramatically improve travel in Massachusetts.