"Investigators identified several issues, including sparks from equipment and flashes of electricity from insulators, which keep the electrical third rail separated from the ground. The insulator flashes, or "flashovers," happened on Sept. 29 in the Transbay Tube, Oct. 20 at Embarcadero Station, Nov. 25 at Civic Center and Dec. 8 at Montgomery Street. Sparks were reported at the Montgomery Station on Aug. 29, at Lake Merritt Station on Sept. 20 and in the Transbay Tube on Oct. 19."
"Investigators determined that a combination of causes were behind each disruption, with the most commonly cited being traction power issues and environmental factors. The "environmental factors" include dust from the grinding of track rails inside BART's tunnels and puddles along tracks and higher humidity in underground stations. For example, some of the insulators had black residue and metal loss, according to the report. The agency had previously said that debris or dust caused an insulator to blow at Civic Center, leading to the Nov. 25 disruption."
Nine major service disruptions between Aug. 29 and Dec. 8 occurred between West Oakland and Civic Center stations, including three inside the Transbay Tube. BART staff, third‑party investigators and experts logged about 15,000 review hours, visited sites at least 37 times and completed more than 180 vehicle inspections. Identified issues included sparks from equipment and insulator flashovers that separate the electrical third rail from ground. Investigators found combined causes: traction power problems, environmental factors (track‑grinding dust, puddles on tracks, higher underground humidity), and human or external objects like aluminum cans and Mylar balloons. Insulators showed black residue and metal loss; poor contact and rough, contaminated surfaces increased electrical arcing.
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