Update, 12:30 p.m. Thursday, June 27: BART said Thursday that it's still investigating an incident earlier this week in which one of its new trains shut down, forcing 421 passengers to exit through a tunnel in downtown Oakland.

The agency says it has determined the problem Monday began with malfunctioning air compressors on two adjacent cars on the eight-car train. That problem disrupted communication between the train's lead "cab" car and the two cars with the compressor problem, BART said in a brief statement.

"This resulted in the train failing in a safe mode and being rendered unable to move," the statement said.

BART spokesperson Alicia Trost said in an email that technicians continue to run diagnostic tests on the two malfunctioning cars. The train's other six cars have been returned to service.

The vehicles, which cost about $2 million each, are part of the new fleet BART began introducing into service last year.

Original post, Monday, June 24:

BART is recovering after one of the transit agency's new Fleet of the Future trains broke down Monday morning in downtown Oakland, snarling service in all directions and forcing more than 400 passengers to detrain and walk through a tunnel to a nearby station.

BART spokeswoman Anna Duckworth said the incident -- the cause of which hasn't yet been diagnosed -- began about 10 a.m. when an eight-car Richmond-bound train ground to a stop between the Lake Merritt and 12th Street stations.

Duckworth said technicians tried for more than an hour to troubleshoot the problem without success. The agency then made the decision to detrain the 421 passengers aboard the run and have them walk "a few hundred feet" to the platform at 12th Street.

"When they couldn't get the train going again and the time kept creeping and it became an hourlong delay, we decided it was time to get these passengers off the train," Duckworth said.

BART deployed a "rescue train" to the downtown Oakland tunnel, which was coupled to the disabled train and towed it at 10 mph to a pocket track near MacArthur Station.

The net effect on BART service: A massive backup behind the disabled train that caused major delays systemwide, with some passengers forced to get off trains that were turned back in the middle of their routes.

Some passengers on the stalled train complained that BART didn't do enough to communicate what was happening during the hour-plus before they were told they'd be taking a hike through the darkened trackside passageway.

Marisol Medina-Cadena, a freelance radio producer, was one of those stuck on the stopped train.

"There were no announcements," Medina-Cadena said. "... It seemed like the speakers might be down and there was no way to make an announcement."

Bevan Dufty, the president of the BART board of directors, said in a tweet Monday afternoon that the public address system had, indeed, failed.

Medina-Cadena said that there was confusion, too, about the order to leave the train. At one point, she said, a BART employee she believed to be a technician told people on her car that "Hey, we might have you guys walk through the tunnel."

"So we just followed him," Medina-Cadena said. "But none of the other cars got that announcement, so they were all looking at us like, 'Where are you all going?' And we were like, 'Wait, didn't you get the announcement that we might be walking the tunnel?' There was no coordinated communication at all."

BART spokesperson Duckworth said the agency's technical staff had not determined the cause of the breakdown as of midafternoon Monday.

Quoting staff, she said, "We won't have an answer until we take the cars to one of our shops for inspection and diagnostic. No time has been scheduled yet for the train to be moved from MacArthur Station."

BART currently has taken delivery of about 80 Fleet of the Future cars, which cost about $2 million each. The agency hopes to acquire a total of 1,081 new cars from Montreal-based manufacturer Bombardier.

They are a key piece of BART's plan to increase rush-hour passenger capacity through the Transbay Tube by 45 percent by 2027.

BART and Bombardier announced plans earlier this month to move final assembly of the new cars from the company's plant in upstate New York to a facility in Antioch.