While BART has brought in engineers to probe a recent rash of smoky mishaps in the Transbay Tube, transit agency officials acknowledge to NBC Bay Area’s Investigative Unit that it gave up five years ago on cleaning dust its own officials blamed in at least two prior “flashover” incidents in the subway.

At the platform at BART’s Rockridge station in Oakland, riders can easily see some of the 100,000 insulators in the transit system. The ceramic devices serve a vital function – they keep power from flowing from the high voltage third rail into the ground.

When clean, they are normally white or gray, but over time, brownish iron dust is inevitably deposited by the action of passing trains.

At Rockridge, that caked-on layer is on full display. When tracks are outdoors, the layer is normally

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