the 1920s on the Philadelphia-Washington, D.C. main line between Philadelphia and Wilmington, and on the West Chester Line between Philadelphia and West Chester, with the latter two lines being fed through a single substation located in Chester. New York - Washington Extensive electrification after 1925 occurred on the PRR's New York-Washington line (now part of the Northeast Corridor), the Chicago-Philadelphia Main Line between Paoli and Harrisburg, several major commuter lines in Pennsylvania and New Jersey, and on major low-grade, through-freight lines, including the Trenton Cutoff, the Atglen & Susquehanna, Port Road, Philadelphia & Thorndale, Shellpot, and Enola branches. All electrification done after 1919 used the same catenary supports used on the Paoli commuter line, but with the catenary being supplied with "transmission" lines with the voltage stepped-down at substations located every . The PPL-owned Safe Harbor Dam, located near the Exelon-owned Peach Bottom Nuclear Power Plant between Conowingo, Maryland and York, Pennsylvania, supplies the power for all post-1925 electrical expansion projects, while Exelon supplies the pre-1925 electrification areas through the existing Philadelphia, Ardmore, and Chester substations. Plans were made in the thirties to extend electrification to Pittsburgh, but were not pursued due to the Great Depression. Since its takeover by Amtrak in 1976, both the Northeast and Keystone Corridors are undergoing extensive wire replacements, either by Amtrak or SEPTA, while the through-freight branches taken over by Conrail have been de-electrified and freight operations carried out by diesel locomotives. Those lines that were de-electrified, but have transmission lines are maintained by Amtrak through arrangements through Conrail's successors, Norfolk Southern and CSX Transportation. Rock Island & Southern Railway This railroad electrified between Rock Island and Monmouth, Illinois using an system. Spokane & Inland Empire Railroad In 1906, this railroad electrified from Spokane to Colfax, Washington and Moscow, Idaho using a system. Suburban Commuter Operations Boston, Revere Beach and Lynn From 1928 to its abandonment in 1940, the narrow-gauge Boston, Revere Beach and Lynn Railroad was operated with overhead electrification. The railroad shut down in 1940, and portions of it were used for the MBTA Blue Line which opened in 1953, although it used an entirely new electrification system and is operated as part of a subway system. Caltrain The Caltrain Modernization Program (CalMod) is a $1.9 billion project that will electrify the railroad's main line, which serves cities in the San Francisco Peninsula and Silicon Valley. Caltrain service has existed almost unchanged