It’s an engineering mystery. Last month, Bay Area Rapid Transit (BART) trains traveling east under the San Francisco Bay began failing due to damage to electrical propulsion equipment. The problem dramatically escalated, and on Monday, 29 February alone, 40 cars went down. By the end of that week, according to the San Francisco Chronicle, BART had 80 cars out of service for repairs. Frustrated commuters packed into the remaining, working cars on shorter than usual trains.
At the time, BART engineers said the evidence pointed to some kind of power surge, but couldn’t identify the cause, leading media to call the mysterious cause the “tube gremlin.”
Possible suspects for the power surges, beyond gremlins, included a project done last summer that involved replacing 3658 meters of rail and cleaning of 300 insulators, and a new power substation recently installed near the Transbay Tube. Earlier this month, BART tested one of those theories: it shut down the substation. That move appeared to assuage the tube gremlin, and the problem disappeared for two weeks. At the time, though, BART spokesperson Jim Allison indicated that engineers were still not sure that the problem came from the substation, despite the ensuing calm on the tracks.
And yesterday, Wednesday, 17 March, the gremlin reemerged—outside the tube. Unexplained power surges damaged 50 cars traveling on a section of track east of the bay, between the North Concord/Martinez Station and the Pittsburg/Bay Point Station. Desperate BART officials shut that section of track down, resorting to transferring passengers past it by bus.
The problem, according to the San Francisco Chronicle’s 4 March story, appeared to be limited to the “C” cars, the “newer” cars that date back to the mid-1980s. According to a report from BART [PDF], the C cars use four DC motors per car; the A and B cars, which don’t appear to be affected by the current problem, use induction motors.
As of this morning, BART officials have reported that all inspections conducted yesterday came up normal, and that they are flying in a team of experts from around the country to try to pinpoint the origin of the electrical problems.
Spokesperson Alicia Trost said, "What's occurring is that when a train is traveling over that section of track (between the two stations), it experiences a high spike in voltage and that is damaging a piece of the propulsion equipment on the train car," she said. "There's no safety risk or concern for any of the passengers, even the ones riding on that exact car that's having the propulsion equipment failure. But we have shut down that section of track because we can't afford to damage any more train cars; we already have a limited number of cars in our fleet.”
The company is providing regular updates on its efforts to identify and fix the problem via Twitter, earning kudos on its openness. BART staff members are also using the Twitter account to express their own frustration.
“Our system was built to last about 45 years and we've reached that limit,” said one tweet.
“We have 3 hours a night to do maintenance on a system built to serve 100k per week that now serves 430k per day,” said another.
I’d love to hear your thoughts on this engineering mystery in the comments below, and will share them with BART officials.
Update 18 March: A few more specifics emerged on exactly what is failing. BART mechanics reported to the San Francisco Chronicle that the surge is reaching up to 2000 volts, which is twice normal. The surge is causing a $1000 thyristor to fail on the C cars; on the other cars, some fuses have been affected.
Tekla S. Perry is a senior editor at IEEE Spectrum. Based in Palo Alto, Calif., she's been covering the people, companies, and technology that make Silicon Valley a special place for more than 30 years. An IEEE member, she holds a bachelor's degree in journalism from Michigan State University.