A friend of mine and I walked in from the Hudson end of the Sudbury Annex to check it out for that purpose about 2 years ago. There is almost 4 miles of straight track in there. About 1.5 miles in, one rail was missing for 15-20 feet. I don't know how it was taken out since there are no roads nearby, but it was gone. We were also stopped by someone official (ranger?) who wanted to make sure that we were not planning to ride any dirt bikes or 4-wheelers in there.
A quiet motor car might be able to get away with it for a while, but you would have to do allot of brush cutting, and haul in pieces of rail from the ends to patch any holes. This track is owned by the MBTA. They might not be very friendly to you if you are caught.
Engineer999