Nope...all diesel. They use several flavors of original F40PH's and some MK clones, plus a (troublesome) batch of rebuilt ex-CN GP40's, now known as GP40MCs. Try this website, scroll down past the subway lines and you'll find an equipment roster:
http://members.aol.com/rtspcc/roster/MBTAroster.html
There are no current plans for electrification. As of today, only the Attleboro/Providence line has catenary, courtesy of the NEC electrification project. The T has stated frequently that it will not purchase either dedicated electric locomotives or MU's that can operate on only one line.
Boston has had plans for nearly a century for a "North-South Rail Link" that would connect the north and south side, similiar to what was done in Philly in the 1980's. However, don't expect to see this ANY time soon, given Boston's recent history with large public works projects. If it ever came to pass, it would probably trigger signifigant wiring of the CR system, but, like I said, it'll be snowing in that place down south before the NSRL ever gets built.
(Although, a railroad tunnel with 25kV catenary through it would probably put on a good light show if it were done to the same standards as the "Big Drain" we've got now!)