BART has a safety system that can detect the presence of trains based on the electric potential across the tracks (since the wheels are metal). There was a col

BART has a safety system that can detect the presence of trains based on the electric potential across the tracks (since the wheels are metal). There was a collision back in the 1970s or 1980s because the repair vehicles had rubber wheels are were therefore not visible to the safety system. A BART train collided with one of the repair vehicles and killed one of the repairmen.