A root cause analysis of a string of smoky incidents and fires inside the Transbay Tube last year concluded that one likely contributing factor was BART’s failure to reactivate an electrical surge projection system that had been deactivated during seismic retrofit work in the underwater tunnel. The root cause analysis covered eight mishaps that began in August of 2025 and continued until the end of the year. One likely contributing factor, the analysis said, was that the tunnel’s power surge protection system had been deactivated during many of the incidents. During a six year seismic project, BART removed so-called cross bonds – wires that function to share power by linking the two sets of tracks running through the Transbay Tube.The bonds were removed to allow workers to move safely around rails that would otherwise be energized. But after that work was done in 2024, BART left the system disconnected, the consultants found. Without the power being shared with the cross bonds, the system could be more vulnerable to the string of still-unexplained power surges that began in the summer of 2025. Last November, following the first of what became a total of eight subway incidents, BART restored the disabled power balancing system. And at a BART board briefing in January, BART General Manager Bob Powers took responsibility for not reviving the disabled system. “They should have been put back with that project, and there’s no two ways about that. And we’re owning that,” Powers said. “Those bonds should have been put back in.” Beyond the deactivated power balancing system, experts had highlighted another possible factor in all the mishaps — BART’s failure to clean its insulators – ceramic devices designed to block power surges from flowing from the high voltage Third Rail to the ground. But the independent analysis did not point to insulator failure as a root cause of the tunnel mishaps. BART initially identified a blown insulator in the first of the incidents in the Transbay Tube, when smoke filling the tunnel forced a train to halt on Aug. 29. Blown insulators were identified in several of the subsequent incidents. Back in 2015, BART blamed the build-up of metallic dust on subway insulators as causing flashover incidents. The agency ordered the ceramic devices cleaned of the dust known to cause short-circuiting and fires on insulators. But in 2020, BART stopped cleaning its insulators, citing the risk that they could become damaged from the dry ice crystals used to clean them. Although the consultants in the report on the more recent events discounted blown insulators as a root-cause, their findings indicated environmental conditions – potentially including caked on dust — may make insulators more susceptible to failing. The team’s inspection of the blown insulators found “normal aging and environmental exposure expected within tunnel installations,” according to the summary of the findings. By this year, BART had resumed cleaning dust from the insulators using soap and water. Separately, the analysis identified another act by BART that may have played a role in the incidents. It noted that BART “disabled” aging sensors designed to protect the system by shutting down circuits during power surges. BART turned off the advanced protection measures, however, hoping to avoid nuisance trips of the breaker system, as a result, “relying only on basic breaker protection,” according to the study. The outside engineers noted that BART has since upgraded that circuit protection system by installing new equipment to better protect the system against power surges. Still, the final report summary to the BART board says the agency continues to face a challenge in setting protective systems to both protect against fires and prevent “nuisance” trips. ...read more read less