One of my favorite (and productive) sites to metal detect over the years has been abandoned railroad lines. Sites can include bunk houses for workers, small hotels and stores situated along long-forgotten routes or supply bunkers built and left to stand the test of time. Finding these old lines involves some research, but the rewards far out-weigh the time involved in finding them.
As far as documenting railroads that are no longer in use, there is really only one book that I would recommend based on it’s content and usability. Written by Waldo Nielsen, it’s title is Right-of-Way; Guide to Abandoned Railroads in the United States.
This is a well-written book that discusses the different periods in the growth and demise of railroads in the United States and then provides a state-by-state listing of railroad spurs and even entire lines that no longer exist. Each state’s listing is accompanied by a map showing where each line was located. I have personally used this book on find three railroad spurs that were abandoned in the early 1900′s, and in each case, the book in combination with a modern topographical map easily pinpointed their locations. Hiking along what remained of the old bed (the tracks had long since been taken away), my wife an I found three switching sidings, an old bunkhouse, and what was left of a round house. A few hours of metal detecting these sites produced a number of coins dating back to the mid-1800′s and several nice railroad collectibles.