@gemini Yeah, these days if we were going to spin up a new line it'd be good if it didn't use the green line tracks, as those are clogged enough as it is.
I suspect we would be much better off with more light rail rather than more heavy rail, for a variety of reasons, though if they electrified the commuter rail line from whichever station it starts in to out past Newton that would be pretty nice. As would taking the storrow/soldier's field route and going on into watertown by the arsenal.