If you want to do Boston-Montreal, realistically you're just going to run it out through Springfield and then north on the existing Vermonter/Valley Flyer route, letting you get more value out of planned/intended investments on those corridors for other services, higher/more useful frequencies, and valuable connectivity at the expense of an indirect routing.

The old line wasn't particularly fast when it did exist and while a BOS-SPG-WRJ-MTL routing won't be faster than the historic timetables had for the old line (which were ~2hrs WRJ-Concord, ~4hrs WRJ-Boston on an express making very few stops), it won't be a lot slower either.