The Times says:
The fire occurred at 7:05 p.m. along the tracks spanning Interstate 680, and was believed to have been caused by an object hanging from a San Francisco-bound train that touched the third trail and sparked, Allison said. Witnesses described the sparks as looking like a Fourth of July display.
The tracks between Walnut Creek and Lafayette were shut down for about two hours, temporarily stranding two passenger-filled trains in addition to the train that sparked the fire. Power to the track segment was restored long enough so they could continue to the next station, and then was turned off again, Allison said.
Contra Costa County firefighters contained the small fire at 8:05 p.m.
"It's too early to tell" when service on the second track will reopen, BART spokesman Jim Allison said. "But delays shouldn't be a problem."