"A special team of experts, from inside and outside BART, searched for the cause of the problems, first looking at trackside equipment that provides power, then examining the cars themselves.

While the problems eventually disappeared on their own, the experts believe that metallic particles, from rails ground by BART crews to reduce noise, may have been the cause. The particles, they think, clung to the underside of BART cars and caused electrical arcing. The issue seemed more likely to occur when the ground was wet and on the uphill climb between the two stations, when the trains are forced to use more power, Allison said.

The agency has since started using vacuum trucks to suck up metallic particles created by rail grinding in the area, he said, as well as picking up debris along the tracks."

Experts? Per their website, BART third rail is 1,000 volts DC. Even at that voltage, DC really doesn't like to jump compared to a higher voltage or AC. Even if it did somehow "flash" due to steel dust from rail grinding, how would the voltage increase and not trip the ground circuit? Surely they have recordings of the substation outputs to see if it is a failure of the static rectifier system? So do the substations reduce the voltage through transformers the high voltage AC input to the needed voltage and then rectify it? Or is it some kind of chopper system subject to leakage? Is there a transient voltage in the power system? That would be the biggest suspect in my limited experience.