WWLP-TV, Channel 22 in Springfield, MA, 3/20/17:
Green Line weighs heavily in MBTA’s long-term fleet plans
Gov. Charlie Baker has repeatedly emphasized the need to focus on the MBTA’s nuts and bolts infrastructure problems, improving signal systems, track and third rail.
This article hints towards a possible merge between coaches and locomotives, DMU's. But while DMU's might be appropriate for a shorter line like the Fairmount Line, they might not be good for the longer routes like Haverhill, Worcester, and Providence.
A similar question faces officials considering whether to purchase diesel multiple units or electric multiple units – a kind of cross between commuter rail and subway cars that could run on the Fairmount Line or other future inner-city routes.
DMU's yes, but I doubt the MBTA will electrify their system outside of what runs on the Amtrak line to make EMU's worthwhile. And the following clarifies things more. Both locomotives and coaches are mentioned:
On Monday, board member Steven Poftak told his colleagues the vehicle fleet plan under development will likely include “an almost full replacement of the Green Line” trolleys, and he said replacements are needed for 50 commuter rail locomotives and upwards of 200 commuter rail coaches in the coming years.