The average New York City subway car travels roughly 53,000 miles per year across some of the oldest transit infrastructure in the world. Rumbling through a web of grungy tunnels and weather-beaten elevated tracks, the cars are subjected to overcrowding, underfunding, vandalism, garbage and routine wear and tear.

Sometimes it’s a marvel the system functions at all.

Breakdowns do occur, but the number might be a lot higher if not for a diverse legion of technical specialists who keep the moving parts doing just that — moving.

Every single wheel, motor, brake, axle, wire and door on every subway car gets completely refurbished every six to 12 years at the Coney Island Overhaul Shop in Brooklyn or its sister facility at 207th Street in Manhattan. The work is part of a scheduled maintenance program, introduced in 1989 and designed to prevent breakdowns before they occur.

The system works much like a scheduled tuneup on an automobile. Trains roll into the facilities, where they are disassembled, reconditioned to near-factory specifications, and then sent back out into a railroad that functions as New York’s vascular system, pumping more than a billion riders across 345 million miles each year, 24 hours a day, every day.