In the early 1850s, the massive front of the Allegheny Mountains, standing 2,161 feet above sea level, blocked westward advance. This obstacle culminated in the creation of the Gallitzin Tunnels and the Horseshoe Curve, both of which were dug out of near-impenetrable geographic formations. Using switchbacks, excavations, and pure innovation, engineers reduced grades and effectively conquered the mountains. To conduct these laborious endeavors, the Pennsylvania Railroad hired job-hungry Irish immigrants. The hazardous work lasted three grueling years.

The end result was nothing less than monumental. The Curve became known as one of the eight engineering marvels of the world. The completion of the Curve was widely celebrated and heralded throughout the state as a grand opportunity. The now-iconic railroad link opened for business on February 15, 1854. Over the next century-and-a-half, the landmark also became a tourism destination, a target of Nazi spies, and one of the primary east-west arteries of railroad travel in the nation.

https://www.railroadcity.org/curve.html