What would San Francisco do without BART? Have a quieter commute, for starters.

For all that the Bay Area depends on the workhorse rail network, the signature, nightmare-inducing noise of train wheels screeching on rails is nobody’s idea of a rush-hour treat.

In 2017, the agency promised to begin converting trains to a newly designed wheel shape that’s supposed to banish that banshee wail, writing on the BART blog:

By slightly tapering the wheel profile using the latest simulated modeling techniques, we hoped to reduce metal-on-metal contact and its consequences. The new, reduced-contact wheel profile has shown as much as a 15 decibel decrease in interior noise on the current fleet. Fifteen fewer decibels may not sound like a big jump, but remember that decibels—like the Richter Scale—are measured logarithmically, not linearly, [...so] a 15 dB decrease is many times quieter than before

In February of this year, BART posted an update claiming that more than a third of its “legacy fleet” (i.e., all of the cars except for the new, Bombardier-designed ones) had converted to the new design.

And last week, BART engineer Ben Holland announced on BART’s podcast (yes, BART produces a podcast) that nearly 55 percent of the conversion jobs are now down, and that “we expect to be over 90 percent by December of this year, 2018.”

Holland and crew actually have to cut all eight of the wheels on a BART car into the new shape one by one. He does his best to explain why the new wheel is quieter, although it’s a little opaque:

“The original [wheel] was like a cylindrical section of pipe. The new profile is tapered, like a curved barrel. [...] With the conical-shaped wheel profile the wheel set actually finds its own center where the inside wheel will find a smaller radius on the taper and the outside wheel will find a larger radius on that taper and you’ll actually get some nice steering out of that. “That’s how we hope to and so far, have shown to reduce a lot of wear as a result of the new profile unlike the cylindrical profile which again just steers with the flange basically rubbing against the edge of the railhead.”

Streetsblog puts it much more plainly: “BART’s cylindrical wheels [...] generate a corrugated pattern on the rail surface. That pattern acts like a violin string and generates BART’s terrible howling sound.”

In 2011, a San Francisco State study in the peer-reviewed Journal of Urban Health warned that while the risk was low because of the short nature of BART trips, there was “evidence of potential noise exposures that may be deleterious to the health of BART passengers” from wheel grind, so the switch comes none too soon.