Getting back to Walt's mention of the Baltimore & Annapolis, interurban archeologists in the Baltimore-Washington area can see a lot of relics of the onetime Washington, Baltimore & Annapolis high-speed mainline between Baltimore and Washington.
Starting in Baltimore, the WB&A's original (1908) terminal and general office building still stands in the triangle between Liberty St. and Park Ave. south of Lexington. Its north-end substation exists at Ostend and Scott Sts., at the point where northbound WB&A trains descended from the long trestle paralleling the B&O mainline and entered Scott St.
In Westport, the WB&A's abandoned tunnel exists alongside the west side of the B-W Parkway at Annapolis Rd. The south portal is plainly visible from the highway. A bit south of there, the underpass under the ex-B&O's Curtis Bay branch is still there -- not to be confused with the nearby light rail underpass, which was originally built for the Baltimore & Annapolis Short Line in 1908, abandoned in 1935, and reopened for the light rail.
The WB&A mainline went directly through present BWI airport, but south of the airport WB&A Road is laid on the right-of-way as far as Odenton. At Odenton, the original WB&A shops are still there, now part of a larger (and, I think, recently closed) manufacturing complex. An ex-PRR industrial spur to this plant follows the former Annapolis & Elk Ridge, later part of the WB&A, from Odenton to the plant. En route it passes the site of the WB&A's Naval Academy Junction station, one of the line's busiest points, where Baltimore and Washington passengers transferred for Annapolis.
South of Odenton the line can be followed either on paths or roads built over sections of the trackbed. I haven't ventured that far south yet, so my knowledge pretty much ends there.
Then, of course, there's the B&A hiking/biking trail, which is laid on the bed of the former Baltimore & Annapolis Short Line, which was part of the WB&A between 1921 and 1935, and then became the Baltimore & Annapolis RR. (The original ASL and WB&A paralleled each other between Linthicum and the present Patapscoi Ave. light rail station. After their merger, most of the redundant ASL track was abandoned, but light rail riders can see the piers of the former ASL Patapsco River bridge alongside the light rail bridge -- which at this point uses the WB&A alignment.) As Walt mentions, the present light rail line uses portions of this line and the WB&A right-of-way between Westport and the Cromwell terminal.