The South Mountain Railroad ran along the bank of the Letort and the current hiking trial follows its old bed. Tracks were removed sometime in the 1950s or 60s. Construction began in 1869 and was completed in 1870. The route began at Pine Grove Furnace and passed through Laurel Forge, Mt. Holly, Mt. Holly Springs, across the Yellow Breeches at Craighead, then to Bonny Brook and the Letort then finally into Carlisle where it joined the Cumberland Valley Railroad. It was used mostly to transport ore from Pine Grove to Harrisburg and beyond.