Peter Fairley, writing for IEEE Spectrum:
DC endures in San Francisco because more than 900 of PG&E’s customers still need it. Most of the utility’s customers transitioned to AC lightbulbs and appliances easily enough as competing power distributors coalesced within PG&E and harmonized their equipment around AC. But for some of these building owners, however, elevators were a problem.
DC-driven winding-drum elevators—the leading design until the 1930s—use a DC motor in the basement that winds and unwinds the elevator’s steel cable on a steel drum, thus lifting and lowering the car from pulleys atop the elevator shaft. DC drive was the only way to go at the time for a speedy elevator, because only DC could deliver variable-speed operation for smooth starts and stops. The DC motors were also energy efficient, capable of something that has only recently become possible with modern elevator designs: regenerating power when the elevator descends.
I love these kinds of stories. What other infrastructure is hidden above our heads and below our feet?