Here are a couple of really arcane relics that only the hardest-core RR archeologists would want to explore:
First is the famous, or infamous, South Pennsylvania Railroad -- the projected mainline across southern Pennsylvania between Pittsburgfh and Harrisburg. This line, as most historians know, was William H. Vanderbilt's planned invasion of the PRR's mainline territory, and was to include 9 tunnels as well as much heavy grading through the succession of mountains and valleys en route. Work was stopped when it was about 60% complete, and the route eventually became the route of the original Pennsylvania Turnpike.
Many people assume that the Turnpike was built on the old rail alignment, but this isn't so. The Turnpike completed seven of the tunnels, but otherwise it used more direct alignments, since it could use a 3% maximum grade instead of the railroad's standard of 1%. So all through Fulton, Bedford, and Somerset counties one can find evidence of the line's grading, including cuts, fills, and culverts. One of the better-known relics is a large fill at Geiger, PA, three miles north of Somerset, where the SP crossed the B&O's Johnstown branch. At this point the SP had to build a short tunnel for the B&O to pass under its fill, and the tunnel still remains.
The other is the so-called "Tapeworm" railroad (it never had a formal name), built in the late 1830s by the State of Pennsylvania at the behest of the politician and entrepreneur Thaddeus Stevens. This line was to run from Gettysburg to a connection with the B&O somewhere west of Hagerstown. (At that time the B&O was planning to follow the north (or east) bank of the Potomac.) It was graded and almost all its stone bridges and culverts were completed between Gettysburg and the west side of Jacks Mountain. Like the South Penn, work was then stopped.
The Western Maryland took over the route in the 1880s and used the right-of-way between Gettysburg and Orrtanna, PA, including the original 1830s-era stone bridges -- all of which remain in CSX service today. Between Orrtanna and the summit of Jacks Mountain, the WM used a different, and steeper alignment, while the Tapeworm route followed a more roundabout path generally to the west and north of the existing line. Several large stone culverts remain on this route, and in wintertime the two grades can be clearly seen on the east side of Jacks Mountain.