EMU's are too different a car fleet to bring in and maintain as all-new. Other RR's like Metro North and LIRR do it because EMU's have been running on those tracks since the day electrification came in the early 20th century. Back when your only locomotives were steam or less-nimble electrics pulling coaches than there are today. Those RR's have way more EMU's than they have push-pull coaches. To introduce electrics on the T there's not going to be any way to justify the startup cost unless they have complete and full access to their existing entire fleet of coaches. Which means this is going to be an electric loco + standard coach system. Electric locos accelerate/decelerate far better than diesels, so it's not like there isn't a whole lot of dwell time improvement to be had with a standard push-pull configuration headed by an electric. The upside of EMU's over electric locos isn't so drastically higher that going for a total draw-down and replacement of the coach fleet would ever pay dividends. Also, the T doesn't have the kind of stop density and headways on CR that New York has. The New Haven Line is almost a quasi-subway with how incredibly dense it is. That maxes out the advantage of EMU dwell times vs. electric push-pull far more than the T ever would.
DMU's are a little different matter because the accel/decel difference is more dramatic on diesel lines, and in the case of the Budds the cars could seamlessly run in push-pull as unpowered coaches or as powered MU's for more flexibility. When their engines were kaput they simply got repurposed for years on end as coaches instead of getting scrapped. Unfortunately I don't think there's a modern build like Colorado Railcar that can do that so seamlessly, and on the EMU side doesn't look like there's much today that can do multipurpose as well either. That makes it difficult for the T to justify buying MU's for just the Fairmount. It would be a lot easier if there were more short-turn lines with denser station stops, like doing the Reading Line, 128 on the Fitchburg, Riverside or Framingham on the Worcester, Salem on the Eastern Route, Needham with an MU fleet and denser inner stops. Sort of an "Indigo Line" network for all the inside-128 centric service. And if somebody took the near-perfect Budd design and modernized it with new systems and engines...sort of a "PCC the Budd" effort to recycle the good stuff in the design and make an affordable modern standard built by lots of vendors. Maybe even with EMU flavors of the same design. But until something with that low a barrier to adoption comes along, diesel or electric push-pull is the only right-sized configuration at all realistic for the T. Our push-pull CR system is conventional enough and has capable enough equipment for "stretching" service density that it doesn't begin to approach the threshold for going MU on a wide scale.
DMU's are a little different matter because the accel/decel difference is more dramatic on diesel lines, and in the case of the Budds the cars could seamlessly run in push-pull as unpowered coaches or as powered MU's for more flexibility. When their engines were kaput they simply got repurposed for years on end as coaches instead of getting scrapped. Unfortunately I don't think there's a modern build like Colorado Railcar that can do that so seamlessly, and on the EMU side doesn't look like there's much today that can do multipurpose as well either. That makes it difficult for the T to justify buying MU's for just the Fairmount. It would be a lot easier if there were more short-turn lines with denser station stops, like doing the Reading Line, 128 on the Fitchburg, Riverside or Framingham on the Worcester, Salem on the Eastern Route, Needham with an MU fleet and denser inner stops. Sort of an "Indigo Line" network for all the inside-128 centric service. And if somebody took the near-perfect Budd design and modernized it with new systems and engines...sort of a "PCC the Budd" effort to recycle the good stuff in the design and make an affordable modern standard built by lots of vendors. Maybe even with EMU flavors of the same design. But until something with that low a barrier to adoption comes along, diesel or electric push-pull is the only right-sized configuration at all realistic for the T. Our push-pull CR system is conventional enough and has capable enough equipment for "stretching" service density that it doesn't begin to approach the threshold for going MU on a wide scale.