NBC Bay Area has learned last week's slow-speed BART crash was triggered when the train car's electrical system interfered with the train car's braking system during testing.
BART is expected to announce details of its investigation on Wednesday, but sources tell NBC Bay Area that the electrical system glitch undermined the brakes, causing the car's crash into sand berm Friday afternoon.
The new car was going about 10 mph when it ran off the end of the test track.
The car arrived earlier this month at BART's facility in Hayward. It crashed during tests of its braking and hydraulic systems.
While the car was not damaged, crews had to laboriously remove sand from its mechanical system and wheels. The testing has been stopped until engineers determine the cause.