The Southeastern Pennsylvania Transportation Authority (SEPTA) operates a 25 Hz traction power system in the vicinity of Philadelphia, that it inherited from the Reading Company. This system is separate from, but similar to, the system built by the Pennsylvania Railroad (PRR), which is now operated by Amtrak. SEPTA's trains can run over either system because the voltage and frequency presented to the locomotive are essentially identical. However, the ex-Reading system is not electrically connected to the ex-PRR system.
SEPTA owns all of the substations described herein. Although SEPTA also owns former PRR substations along the Media/Wawa Line and the Chestnut Hill West Line, as well as a new substation just north of 30th Street Station, those facilities are electrically part of Amtrak's 25 Hz traction power system.