The World Famous Horseshoe Curve, Altoona Pennsylvania

The Horseshoe Curve is a three-track railroad curve on Norfolk Southern Railway's Pittsburgh Line in Blair County, Pennsylvania. The curve is roughly 2,375 feet (700 m) long and 1,300 feet (400 m) in diameter. Completed in 1854 by the Pennsylvania Railroad as a way to reduce the westbound grade to the summit of the Allegheny Mountains, it replaced the time-consuming Allegheny Portage Railroad, which was the only other route across the mountains for large vehicles. The curve was later owned and used by three Pennsylvania Railroad successors: Penn Central, Conrail, and Norfolk Southern.