A little late to the conversation here, but let me see what I can add.
Bluntly speaking, there's no truly legal and safe place to park around Thomas Viaduct's south end. You can ditch a car somewhere and hike, but every place one could reasonably ditch a car is a private driveway or subject to ticketing. There used to be a spot to pull off along the tracks right as that road got to track level, but a guardrail is now in place. And this is February or March--you'll probably have snow in your way to boot. Patapsco State Park and CSX are reportedly working on a plan to make the Viaduct more accessible and visible, but it won't be in place in time for your trip.
Three publicly-accessible areas worthy of note:
Bush Street, just off of Russell Street/I-295 south of the stadiums. At the Bush Street crossing is a parking lot north of Carroll Interlocking with good visibility both ways, and public parking/trainwatching is at least tolerated.
Bailey's Wye, southeast of M&T Bank Stadium (Ravens) and Oriole Park at Camden Yards. There are places in the neighborhood where one should be able (weather permitting) to park a car and hang out. You'll see traffic into and out of Riverside Yard and MARC facility, into and out of Howard Street Tunnel, and CSX and MARC trains (the latter Mon-Fri) from Camden and the tunnel SB to DC and points west. Freight averages 25-35 movements a day, including the occasional transfer or local. Light Rail also rolls through overhead.
On the east side of town off of US 40, Bayview Yard has a parking lot on the north side of 40/Pulaski Highway, next to the Y with the east end of Monument Street, that used to be a popular railfan hangout, as you catch what's going into and out of Bayview and past on the main.
As usual, use your head. A mild bit of trespassing may be tolerated briefly, but don't make a fool of oneself. Trains around Bayview, especially TOFC/COFC, are monitored by CSX and Baltimore City police for potential burglary or vandalism with theft/robbery in mind. There was ongoing trackwork around Bailey's Wye/Carroll which may have things tied up a bit.
CSX had assigned a new genset loco to Curtis Bay, southeast of the harbor. I would not advise heading into the yard proper, but a drive around the vicinity may be fruitful.
A good and public train-watching spot on the Northeast Corridor east of town is the MARC station at Martin State Airport. Amtrak trains will blast through at 125 mph, and there exists the possibility of seeing NS locals to Aberdeen/Havre de Grace and even coal trains to the power plant near Chase and to Delaware plants. Don't let the folks tell you that these trains run only in darkness--I rode an Amtrak Acela train to NYC and back on Tuesday and spotted a mainline mixed freight, a coal train, and two locals between Baltimore and Delaware in daylight.
The B&O Museum and the Baltimore Streetcar Museum are must-sees if you've not seen them; allocate at least a half-day to the B&O Museum to see everything. If you plan properly, your B&O Museum ticket includes the Ellicott City RR Station Museum in nearby Ellicott City. The BSM now has a Philly snowsweeper, so snow won't stop them--open Sundays year-round.
If you like transit, a $3.50 day pass on the MTA gets you a full day of riding on the Central Light Rail Line (which was patched together from both the former PRR from Baltimore to York/Harrisburg and the former Baltimore & Annapolis interurban route from Baltimore to--duh--Annapolis) and the Baltimore Metro (Owings Mills-Johns Hopkins University).
I would be derelict in my duty if I didn't mention that the Baltimore Chapter NRHS, celebrating its 75th anniversary in 2011, meets the second Monday of every month at the Baltimore Streetcar Museum, and is holding its 75th Anniversary Banquet on March 12, with guest speaker Jim Wrinn of Trains Magazine. The Chapter runs a joint Library with the BSM, housed at the BSM, and is open for researchers every Wednesday from 10 AM to 4 PM and at least the second Sunday of every month noon to 5. Check the Chapter's newly-revised website at for updates.
