The commuter rail as-is is too infrequent, too packed and too polluting with all those stops & a diesel train.

Overhead electrification on the commuter rail lines or even battery-electric trains would allow the T to run Green line style trams further out into the 'burbs out to I95 and let the commuter rail trains focus on the I495+ outer burbs/medium distance without having to stop frequently on the inner part.

Reusing the existing commuter rail lines with trams is much cheaper than building new T lines as they did with the Green line into Medford (and if you don't live in Medford, it doesn't help you!).