In Gardner, get off Route 2 East at the first Gardner exit, go under the RR bridge past Dunkin donuts. (Sometimes trains stop behind DD for coffee). Continue on the main road, go straight at the light you come to where the old T platform was, you can usually park there and that is a good spot to watch. It is at the East end of the yard.
Particularly if P&W is there dropping coal or picking up empties and/or interchange, that is the end they are usually working.
Another spot is off Route 31 in West fitchburg there is a curve there which might be a good spot to get pix although I have not tried it.
Another good spot besides Gardner is Ayer where you can park near the T station and there is usually activity in the Yard plus the MBTA locals, as well as the through freights. If you are lucky you might see one of the CSX trains such as SEPO or the autoracks.