Amtrak's 25 Hz traction power system is a traction power network for the southern portion of the Northeast Corridor (NEC), the Keystone Corridor, and several branch lines between New York City and Washington D.C. The system was constructed by the Pennsylvania Railroad between 1915 and 1938 before the North American power transmission grid was fully established. This is the reason the system uses 25 Hz, as opposed to 60 Hz, which is the standard frequency for power transmission in North America. The system is also known as the Southend Electrification, in contrast to Amtrak's 60 Hz traction power system that runs between Boston and New Haven, which is known as the Northend Electrification system.

The Northeast Corridor (NEC) is an electrified railroad line in the Northeast megalopolis of the United States. Owned primarily by Amtrak, it runs from Boston in the north to Washington, D.C. in the south, with major stops in Providence, New Haven, Stamford, New York City, Newark, Trenton, Philadelphia, Wilmington, and Baltimore. The NEC closely parallels Interstate 95 for most of its length. Carrying more than 2,200 trains a day, it is the busiest passenger rail line in the United States by ridership and by service frequency.