BART officials say they are investigating whether insulator failure is to blame for the meltdown Monday that snarled the system for more than three hours during the morning commute.

Spokesman Chris Filippi said the 5:57 a.m. incident began when the operator of a train heard a popping sound inside the Transbay Tube – which experts say is an indication of an explosion that could damage an insulator.

BART had to “single track” -- run trains on one side tunnel only -- until 9:17 a.m., triggering “major” delays, lasting more than 30 minutes.

“The root cause of the incident is under review – when our crews went into the trackway, they did remove a damaged insulator as a precaution,” Filippi said. “A conclusion hasn’t been drawn.”

That incident is just the latest involving ceramic insulators

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