Other natives will fill in more details, but here are a few suggestions confined strictly to the Baltimore area. Washington is another kind of animal.
Basically, if you're tired of CSXT, there isn't much in Baltimore itself for you. NS enters Baltimore from the east over Amtrak's NEC line and has a yard at Bay View in East Baltimore, but any mainline trains (aside from daily locals) run at night. MARC operates on what's called the Camden Line (ex-B&O Baltimore-Washington) and the Penn Line (ex-PRR, Perryville-Baltimore-Washington). Camden Line service is confined to commuter hours; Penn Line trains operate roughly hourly during off-hours and are diesel-powered. MARC electrics (AEM-7s and HHP-8s) show up on Penn Line commuter trains.
Anyway, this is what's around, both CSX and other:
(1) St. Denis station, on the ex-B&O mainline SW of Baltimore is a favorite railfan hangout. There you can see all CSXT traffic in/out of Baltimore plus morning and afternoon MARC commuter runs. (But remember, no MARC mid-day service on this line.)
(2) Within Baltimore, a good AM spot is the parking lot at the point where Bush St. (one-way eastbound) crosses the CSXT mainline. This is about a block west of Russell St. B-W Parkway entrance to Baltimore), but you have to circle around because of the one-way street patterns. Same MARC traffic here and most CSXT freight, although you miss anything to/from Curtis Bay.
Nearby to the north is Bailey's Wye, on Stockholm St. east of the Warner St. crossing. Better for PM photos, and you have an impressive overpass for the light rail line, but parking is iffy, and impossible if there's a stadium event.
(3) A favorite PM railfan hangout is the parking lot off Monument St. just west of Pulaski Highway at Bay View in far-east Baltimore. No MARC here, but you get any CSXT trains on the Philadelphia sub plus local traffic to/from East Baltimore and Sparrows Point. This was busier when GM's Highlandtown plant was operating, but that's now dead and gone.
(4) If you have any interest in electric traction, ride sand photograph the light rail line, which offers a wide variety of operations, from in-street running to lengthy stretches on an ex-PRR mainline (to the north) and former interurban line (Baltimore & Annapolis) to the south.
(5) The nearest short line is the Maryland Midland, based at Union Bridge in Carroll County. They operate a long section of ex-Western Maryland mainline plus parts of the former PRR Frederick branch with sharp-looking GP38s. Excellent scenery, especially climbing the Blue Ridge to a CSXT connection at Highfield, MD. They run daily, but you'd need to consult the experts for a schedule.
(6) For Amtrak and MARC Penn Line trains, probably the best spot in the Baltimore area is the BWI Airport station. There's a large parking garage there, plus free shuttle buses to the airport. The light rail system has a branch to the airport (half-hourly service), so if you want to do it the cheap way, take the light rail to the airport and shuttle to the rail station.
(7) No other railroad museums in the area, but if you have any interest in trolleys, the Baltimore Streetcar Museum is one of the best anywhere. During ythe summer months they operate Saturdays and Sundays with a range of equipment running from a turn-of-the-century open car to a 1930 Peter Witt, to Baltimore's last PCC. Until recently their collection (with equipment that dates back to the 1880s) was confined entirely to Baltimore cars, but recently it has also become the home of several orphan PCCs from Philadelphia. (These are being restored and re-gauged to Baltimore's odd wide gauge, so you won't see any operating.)
OK, I'll let others chime in.
