SAN FRANCISCO (AP) — San Francisco Bay Area residents were scrambling Friday for alternate routes after a regional commuter rail system shut down all its trains due to a computer issue, a second systemwide closure for the beleaguered rail line in four months.

The Bay Area Rapid Transit District’s suspension of service started about 5 a.m. following a computer network upgrade, BART spokesperson Alicia Trost said. The trains were still down as of 8 a.m., and there was no estimate time for service to resume, she said.

Software upgrades are done periodically overnight, but Trost said something went wrong Thursday night that has prevented the agency from starting the computer system that dispatches the trains.

Last Friday, smoke from brakes filled a car after a train suddenly stopped inside the Transbay Tube, prompting officials to temporarily shut down the system between San Francisco and Oakland, the San Francisco Chronicle reported.

In May, the trains shut down for several hours after a power outage caused the train control system to malfunction.

BART carries nearly 175,000 people most weekdays. The system, which connects San Francisco to its eastern and southern suburbs, is struggling to restore ridership that plummeted from pre-pandemic highs of over 400,000 weekday riders.