There's actually some interesting trail options between West Concord and South Acton that are close to, but not on, the MBTA ROW. People are looking into making this ARRT/BFRT connection. If you can get to Old Stow Road where it crosses the Fitchburg Line, there's trails through old industrial lands to River Street in Acton. That would make for a very nice ride. I've done it a few times on a road bike with wide tires, so it's doable but not commute-able now.If you're pushing for a rail-with-trail along the Fitchburg, keep going to South Acton to hook up with the ARRT. Dare to dream big. Might even be doable; looks like there's 20ish feet of ROW on either side of the tracks.