Amtrak's 25 Hz traction power system

The traction power network of Amtrak uses 25 Hz 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 became the standard frequency for power transmission in North America.

Source: Wikipedia — Amtrak's 25 Hz traction power system (CC BY-SA 4.0)