I'm not sure exactly what type of abandoned stuff you're interested in, whether you mean things like old depots, roundhouses, etc., or whether you also mean stuff like bridges and tunnels on old abandoned rights of way. While there are a lot of former depots out there, I'm not sure how many of them are actually abandoned vs. reused, and maybe someone else can fill in this info.
But my own pet interest has been abandoned ROW, and there are a lot of old bridges, plus a few tunnels that I think are pretty interesting. To name just a couple of my personal favorites:
- the old Central Mass tunnel in Clinton, with the west portal right next to Rt. 70 a few hundred feet from the Wachuset Dam;
- on the old B & A Milford branch thru Holliston, there is a neat stone viaduct next to Woodland St., with a short stone tunnel under Highland St. about a half mile away. The rail was just lifted from this route last year;
- along the old "Air Line" route of the New Haven, there are several rusting bridges over streets such as Canal St. in Blackstone, plus what would have been a triple level rail crossing in nearby Millville off of Rt. 122- the P & W underneath (still active), crossed by a truss bridge for the Air Line, next to concrete abutments for the never completed Southern New England railroad;
- an old Bay Colony engine rusting away in Newton on an unused part of the Needham/Charles River line;
- and the old B & M Eastern route bridge over the Merrimac River in Newburyport.
There are a lot of other sites like this throughout the state, but these are just some that I could quickly think of. If you haven't already done so, you might want to check out the "Railroad Archeology" Forum at this site- another good spot.
Hope this helps at least a little.....
- Charlie