Constructed from 1903 to 1906 by the Pennsylvania Railroad as the middle segment of its Low Grade Line, the Atglen and Susquehanna Branch formed part of an approximately 140-mile through-freight route from Morrisville Yard near Trenton, New Jersey, to the Enola Classification Yard west of Harrisburg. Its completion eased freight bottlenecks approaching Philadelphia by separating heavy through-freight traffic from passenger service and local freight on the Pennsylvania Railroad's main line through Lancaster and by bypassing Philadelphia altogether. The project was the Pennsylvania Railroad's largest construction effort to that date, costing nearly $20,000,000 and requiring an estimated 22 million cubic yards of earth and rock to create a freight route linking western markets to the ports of Philadelphia, New York, and Baltimore. To maintain the low grades and gradual curves needed for through-freight operation, the line crossed southern Lancaster County on an exacting course, raising an earthen road across the Chester Valley floor west from Parkesburg and cutting deep canyons through the hills of the Susquehanna River Valley to reach Safe Harbor. On the eastern half of the line, about 80 masonry and steel bridges and culverts carried the route across valleys, streams, and roads. Dedicated for service in July 1906, it operated for decades as an efficient rail corridor meeting eastern seaboard fuel and food demands. The Pennsylvania Railroad's 1938 electrification ended steam locomotion on the line, using hydroelectric power generated at Safe Harbor Dam to supply its catenary system. The line handled World War II freight demands without major changes, but the national decline of rail service in the later twentieth century led to its end, with final freight service on December 19, 1988. In the twenty-first century, new transmission monopoles have been installed along its route, and the 2011 Amtrak Transmission Line Upgrade Project continues supplying electricity from Safe Harbor to the regional grid in support of Amtrak's Northeast Corridor passenger service.