Anywhere along Rt 11, Burnhams Ave is great. CSX and NS lines actually cross at grade at the north end of town, north of Church St near the TBH cement plant, then run parallel before bearing away from each other about 1/2 - 3/4 mile to the south. CSX yard is near Rt 11 and Elgin, not very photo friendly as not much accessibility, NS Vardo Yard is further south, along Rt 632, Maryland Ave. The yard runs parallel to Rt 632 and alot of it can be seen from there. You might also get lucky and see a train from the Winchester and Western, which interchanges with NS. NS is much, much more active here, with alot of hotshot trailer and container trains, some manifest freight too. CSX runs a few locals, one on the Lurgan (runs parallel to Marsh Pike) to Chambersburg, one to Highfield (Hanover Sub, runs parallel to Rt 64, Jefferson Blvd) to interchange with Maryland Midland. Mamifest from the Hanover Sub will arrive between 10am and 2pm or so depending on switching duties along the way. CSX has also recently added container traffic along the Lurgan branch, not sure how much this has impacted frequency. My delivery routes recently changed and I don't get to Hagerstown anymore. Any other info needed let me know, or others here can add.
Gettysburg, stop on 797