Curve

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.