The problem was initially reported shortly after 6 a.m. and prompted service to stop between the Orinda and Rockridge stations, with County Connection buses dispatched to take riders between the two stations, BART spokeswoman Anna Duckworth said.
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As of shortly after 7 a.m., the ventilation problem in the tunnel had been fixed and trains started moving again between the two stations with residual delays, Duckworth said.
The problem followed a power issue also on the Antioch line that was reported earlier Friday at about 5:30 a.m. and affected trains between the North Concord and Pleasant Hill stations until electricians went to the scene and were able to restore service, according to Duckworth.
It has been a tough week for commuters on the Antioch line, which also had delays for several hours Tuesday because of an early morning grass fire near the tracks in Concord.
BART took to Twitter on Friday morning to apologize for the problem that had shut down train service between Oakland and Orinda.
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"We are very sorry for the earlier disruption," the agency said. "Safety always comes first. Ventilation in a tunnel is critical to safe operation."