BART shaved its train wheels into a slight taper to cut its screeching

WIRED reported in September 2016 that after 44 years BART had worked out how to quiet its racket: shave its metal wheels by two millimetres. The chance came from buying a new fleet: when Bombardier won the contract in 2012, BART added an extra request to figure out the awful sound, and Bombardier's German engineers built computer models of the current wheels. Research took four years, three contractors and about a year of testing.

The finding was that it is all about shape. The round wheels sit on a fixed axle, so they don't spin independently; when the train turns, the outside wheels want to go faster than the inside ones, which get dragged. That slipping makes noise and warps the rails, called corrugation, which makes the noise worse. A slight taper lets the wheels re-adjust in bends, says vehicle-systems manager Ben Holland, meaning less dragging, less rail warping and less noise.

Before adopting it, BART had two more contractors test for maintenance costs and safety. It expects the new shape to lengthen a wheel's life from three years to seven. After spending $150,000 on new cutting machines, it simply re-grinds wheels to the new shape during routine maintenance; all 669 cars were to be done within two years. WIRED notes the train will still make other noises.

The cause was geometry, not a faulty part: rigid axles force slipping in curves.

Rail corrugation made each noise worse, so fixing the slipping helps the track as well as the wheel.

Re-grinding at routine maintenance avoids buying new wheels.

The purchase of a new fleet gave a rare opening to ask the manufacturer for a fix.

When buying new equipment, add the nuisance you want fixed to the spec; the supplier's modelling can find a cheap geometric fix.

BART planned to put the new wheels on all 669 cars within two years and on the new fleet from the outset. WIRED noted noise from interlockings, brakes and tunnels would remain.