There oughta be a law. In fact, there is one.
Published: September, 2003
When built, the first General Manager of BART, Brian Stokes, called it "The swift, virtually noiseless and vibration-free electric train." Stokes also promised BART to the Airport and that every passenger would have a seat. While BART has finally managed to get to the airport some thirty years after promised, the commute crowding makes the seat promise just laughable.
In fact, the first BART cars had no provisions for standees and grab rails had to be added in the first months of service. But back to the "virtually noiseless" part, we find passengers holding their ears and wincing in pain as the BART train rockets through the Transbay Tube. Note that plugging your ears and holding on to a grab rail are mutually exclusive activities for humans and thus the BART passenger either must find a door to lean against or risk stumbling against a fellow passenger when electing the ear-plugging option.
So if big, burly guys plug their ears in the tubes, how loud is it? Bay Crossings decided to put a decibel (dB) meter on it and find out. Here is the result: On average straight track, BART runs around 70 dB; on curves, this can rise to 80 dB. Curved, elevated structures and tunnels are louder at around 82 dB. Three spots in the Transbay tube achieve an astonishing 95 dB, one lingering almost three minutes.
The Occupational Safety and Health Administration (OSHA) standard for noise in the workplace is a maximum of 85 dB during an eight-hour shift. OSHA recommends that for each 10dB increase, reduce exposure time in half. So according to OSHA, our BART exposure isn’t damaging our hearing that much. But judging from the pained expressions, it isn’t much fun.
So if Stokes was promising us a "virtually noiseless electric train," it is clear that BART still hasn’t delivered. Bay Crossings wanted to know why not and asked and, unsurprisingly, BART didn’t respond. So Bay Crossings consulted a track engineer, who had some pretty surprising observations.
He noted that BART standards are far more rigorous than a normal railroad. For example, in a curve, BART specifications call for only opening the gauge one sixteenth of an inch no matter the degree of curvature. Real railroads open it more the tighter the curve. The reason is to reduce wheel and rail wear caused by the outside wheel attempting to revolve slower than the inside wheel, thus dragging the wheels along the rail as they attempt to equalize.
Huh? Getting that explained again, the track engineer noted that going around a curve, centripetal force throws the train against the outside rail (called the "high" side due to its elevation). When the wheel is tighter against the rail, due to its shape (like a bell viewed sideways), it is riding higher against the curved edge (called a flange). The higher the flange it rides, the larger diameter portion of the wheel is used, effectively making it bigger.
On the low side, less of the flange is used, making it smaller. The smaller wheel is going around the inside curve that is a shorter distance than the outside. Thus, in a perfect world both wheels are revolving at the same speed, which is important since they are permanently connected and fixed to an axle. In an imperfect world (such as BART), the wheel on the inside isn’t allowed to get to its smaller diameter and it attempts to achieve more revolutions than the high side wheel. Since this is impossible, both wheels protest this abuse. Sometimes loudly.
This oddity harkens back to the days when BART was being built. According to lore, BART spurned the use of any "real" railroad engineering and went out and got engineers uncontaminated by the stodgy railroad thinking of the day. Thus, BART set its standards "higher" than any other railroad and created problems at the same time.
While gauge alone may not be the single cause for noise propagation, the fact is with BART you get lots of noise. So when extolling the virtues of BART, you can tell your friends that BART has one thing that no other transit system has. Noise. And since much of BART is fixed in cement (in tunnels and on elevated structures), fixing the noise is not going to happen any time soon.
