The incident created residual delays throughout the morning.
Around 6:30, as the rush hour was underway, a BART train unhooked inside the Transbay Tube. A metal piece known as a coupler cracked and the nine-car train became a five-car section in front and four stranded cars in the rear.
"This was not a maintenance issue. This was not a human error that we can tell," BART chief spokesman Linton Johnson said Tuesday. "Somehow, the casting of that metal, there was a fracture that started to develop. We don't know when. And, it broke."
The train was headed from Pittsburg-Bay Point station to SFO. Johnson says BART's failsafe system immediately kicked into place to protect riders.
"The computers automatically stop all the trains behind it," he said.
"We smelled some burning, but we're not really sure," passenger Jeff Pizzagoni told ABC7. "But, the stop-and-go were kind of rough."
BART is nearly 40 years old and the first transit system of its kind in the country. But, the coupler in question was new. It was installed on January 2009 as part of a routine equipment upgrades to extend the life of the fleet.
According to Mike Healy, who worked for BART for more than 30 years, trains have come undone a couple times before
"I don't recall all of the reasons why these things happened in the past," he told ABC7. "But, they do happen when you're operating a zillion miles a year."
The damaged piece has been sent to the manufacturer for analysis. The company Wabtec released a statement saying, "We are supporting BART in its investigation of the incident and have no further comment at this time."
BART carries 340,000 passengers a day. The executive director of the Minteta Transportation Institute, Rod Diridon, says his research shows Tuesday's equipment failure was an anomaly that should not worry riders.
"BART is absolutely necessary for our region," he said. "If we did not have BART, we would not be able to operate as an economic unit in the Bay Area."
BART has a $4 billion plan to replace existing trains with 700 new cars. Transit official are currently in Washington trying to drum up the money.