Correct, this is just outside Washington DC, and runs west by the side of the C & O canal. As far as I can make out it was built around 1892 and called the Potomac Valley Railroad, a branch of the Western Maryland Railway. It was eventually closed down in 1959. The track is barely passable due to being thickly overgrown, in particular with the dreaded kudzu vine which is extremely invasive and completely envelops trees, buildings and anything in it path. Its now become illegal to plant the stuff. Check out Wikipedia on the subject there are a couple of great shots.
matsorjc