One of the earliest and most impressive of America's great railroad engineering feats, the Horseshoe Curve was built as a means of overcoming a straight-line grade over the geological feature known as the Allegheny Escarpment or Allegheny front, which separates the ridge-and-valley section of Pennsylvania (on the east) from the Allegheny Front (on the west). Such a straight-line route would have made commercial railroad operations unfeasible from both and economic and technical standpoint.
In an era when roads and canals were the most common means of overland transportation, the Allegheny Portage Railroad provided a novel alternative. The railway carried fully-loaded canal boats over the steep grades of the Allegheny Mountain. The 36-mile system rose almost 2,300 feet above sea level at its summit - the highest level to which canal boats had ever been carried. The project included ten double-tracked inclined planes, powered by steam engines. Its 900-foot Staple Bend Tunnel, cut from solid rock, was the first railroad tunnel constructed in America.