40 years ago, my mother sublet an apartment in a landmark West 67th Street building. At that time, all the power in the building was DC. Fine for lights, hot plates, and DC motors, of which more in a moment. Electronics (like a TV) required a noisy "rectifier," in effect, an AC generator run on the DC power. She kept it in a closet to mute the racket whike watching TV.

The transit system today runs largely on 600 volts DC. This is what NYC Transit, Metro North, and the LIRR send out on their third rails, and what Amtrak and NJ Transit put on their catenaries. The choice is quite rational, since DC is more efficient in turning "electrical" energy (i.e., the power itself) into "mechanical" energy (i.e., a turning motor.

NYC Transit has decided, nonetheless, to slowly convert its rolling stock (on the "L" line, for example) to AC, arguing that the motors are subject to less wear, even though each car must include complex conversion circuitry. Take one of the Transit Museum's occasional tours of the maintenance yards and you'll hear a lot of skepticism about this "improvement."