One benefit the green line has is the ability to split and make up trains on the fly (meaning during a trip, not actually while the train is in motion). No reason a train couldn't split at Brigham Circle, run out to Heath St. separately, utilize both loop tracks simultaneously, and pair back up to a 3 car train at Brigham Circle inbound. I doubt they'd ever do this since it's way too "unusual," but it would solve the issues of the loop, and pretty easily too.
Moderator: Massachusetts Bay Transportation Authority, Brightline Trains
Avatar:3679A (since wrecked)/3623B (now in service as 3636B).