The MBTA is (very very slowly) researching electrifying their commuter rail. The Providence line is a no-brainer, since it is the NEC and is already electrified, so it will probably only take about 5 years, maybe. Next on their list appears to be the new South Coast line, which is an extension of the Stoughton branch of the NEC in Canton, so it is already partially electrified and hasn't gone into service yet. (From the
Project Plan it looks like they are initially going to connect the South Coast line to the Middleboro line instead of following the straight route north to Stoughton. This was an existing ROW but the tracks on some portion may no longer exist and it might require much more work to get it up to standards, but still seems to be the ultimate plan.)
Eventually, they are vaguely thinking about electrifying all the commuter rail lines, including the route of the Downeaster at least as far as Haverhill. So dual-mode (diesel and cat) would make sense, eventually.