PNP264365 Locomotive of the US Military Railroads (Union Army) crossing trestle bridge over the Potomac River (b/w photo) by American Photographer, (19th century); Private Collection; (add.info.: American Civil War railroad; The Union Army crossed the Potomac River (from Washington D.C.) in 1861 and captured the Confederate city of Alexandria, it was occupied for the remainder of the Civil War;); Peter Newark Military Pictures; American, out of copyright.