The likely cause of the incident was a failure of a bearing cage on the gearbox

2  Initial investigations focused on the main motor bracket bolts and the safety bolts, which should keep the motor in its proper place. Faulty safety bolts were blamed for two similar previous incidents at Hainault and Loughton, but in this case these had been checked and passed fit only 48 hours before the derailment.

3  LUL's Final Investigation Report found that the most likely root cause of the crash was a failure of a bearing cage on the gearbox, which in turn caused the motor attachment bolts to break. The motor was not retained by the safety brackets designed to hold it in place.