Head end power
Passenger trains need electricity for heating, cooling, lights, and other purposes. Today, this hotel power is normally provided by the loco, and called head end power, in difference to axle generators in each carriage. If the train is long, several hundred horsepowers might be needed for generating.
The US standard for HEP is 480 volt, 3-phase, alternating current. More details about HEP setups in North America can be found on the website of a company in Portland.