Update, 12:23 p.m.: BART trains are moving again, each packed with hundreds of cranky passengers. The train walker has been detained, according to a BART spokesperson.
BART trains headed to SFO, Millbrae, and the East Bay are stalled in all directions, owing to a person on the trackway between 12th Street and West Oakland. So if you're planning to take BART to work right now, you might want to turn around. Two trains leaving Fruitvale have been stuck on the platform for 20 minutes, according to one marooned passenger.
One of them eventually turned around to dump riders back in Fremont. Another went out of service, leaving passengers stranded at the station.
"This is ridiculous," one woman groused, adding that she'd been stuck on the train for an hour.
BART says it currently has no trains running between the East Bay and San Francisco, but it's working to restore service as quickly as possible. We'll bring more updates as they come in.
Erin Sherbert contributed to this report.