تيار متردد
The subway system began during the war of the currents when Thomas Edison and his opponent, George Westinghouse, struggled over acceptance of direct current or alternating current as the standard way to deliver electricity. Alternating current became the standard for non-railroad purposes, but New York City Subway adopted direct current as more suitable for urban railroad purposes. The companies built their own power stations to generate their DC. To this day, the New York City Transit Authority converts alternating current to 600 V DC third rail to power the trains, as do most transit railways around the world. (The A Division uses 625 V DC third rail.)