Over the weekend, BART did rail replacement work along one of the tube’s tracks. Red and Green line service was paused during the maintenance late Saturday and into Sunday morning, and the agency similarly operated a single track through the corridor, which it said would cause delays of about 10 to 15 minutes.

It’s not yet clear if the work is related to Monday’s outage, Fillipi said.

“We definitely appreciate the patience of our riders on this,” he said. “Even though we do have service to all 50 stations, we do still have major delays, and we know that’s a significant disruption for folks and we take that very seriously.”

Monday’s issues, and the network problems that halted all service for hours in May and September, come as BART faces a “fiscal cliff” and has floated cutting or reducing services if one-time state and federal funding dries up as expected next year. Transit advocates are lobbying to put a sales tax on the November 2026 ballot that would generate revenue for BART and other regional transit agencies also on the verge of major budget deficits.

KQED’s Brian Watt and Dan Brekke contributed to this report.