Four of BART's five lines were halted from about 6:15 p.m. to 6:50 p.m. while electricians cut power to the West Oakland station and removed three metal scraps, spokesman Linton Johnson said.
Riders experienced residual delays until about 8:30 p.m., when all trains were operating on time, he said.
The delays began at about 5:40 p.m. when the operator of a Richmond-bound train reported smoke on the tracks near West Oakland, Johnson said.
BART halted San Francisco-bound service to inspect the tracks but kept trains running east, he said.
Inspectors were still concerned, though, and decided to suspend service in both directions at about 6:15 p.m.
"As painful as it was, we decided to shut down service at the heart of the commute," Johnson said. "It would be a lot more painful if somebody got injured."
Electricians cut power to the West Oakland station and pulled the metal from the tracks.
Riders experienced significant delays of more than 30 minutes system-wide at that time, and trains were still delayed by about 15 minutes an hour after the West Oakland station reopened.
The only relatively unaffected trains were on the Richmond-Fremont line, which doesn't go through West Oakland, Johnson said.
He said investigators have determined the metal did not fall off a train but were not sure tonight where it came from.
The agency tried to use a bus bridge to circumvent the track problems, but BART serves too many customers, Johnson said.
A train car has almost three times as many passengers as an AC Transit bus, and commute trains have eight to 10 cars, he said.
"(The buses) only have so much capacity, and they're in the middle of their own commute," he said.