Con Ed may be shutting down the last of Con Ed's DC service, but almost all of NYC subway trains are powered by 600 VDC on that third rail. Newer subway cars have AC motors and loud converters to convert the 3rd rail DC into AC. That's the sound you hear when those newer trains pull in and out of stations. A quote from nycsubway.org: "Alternating current (AC) operates signals, station and tunnel lighting, ventilation and miscellaneous line equipment. Direct current (DC) is used to operate trains and such auxiliary equipment as water pumps and emergency lighting."