I've ridden the BART in San Francisco a few times but one thing I don't remember about it is how noisy it is. Apparently, it's a big problem (I didn't think so though!) and BART has finally found out why and how to fix it.
Turns out, it’s all about shape. The current round BART wheels sit on a fixed axle, so they don’t spin independently of one another. When the train turns, the outside wheels want to go faster than their inside counterparts, which get dragged along. This inside “slipping” not only makes extra noise—it warps the rails. Called “corrugation,” that damage makes the the goshdarn hubbub even worse.
If the wheels are just slightly tapered, though, they can re-adjust a bit while rounding bends, says Ben Holland, BART’s manager of vehicle systems. That means less dragging of the inside wheel, less warping of the rail, and less noise.
So these BART train cars have been running like ALL WHEEL DRIVE vehicles for this many years?! That's insane.