The runaway train consisted of a shunter hauling sixteen loaded stone wagons, weighing a total of 1700 tones. It had been engaged in marshalling duties with another train in the quarry sidings, when there was a mechanical failure of the handle controlling the train's main air brake. The crew had attempted to stop the train by applying the shunter's direct air brake; however, this was overcome by the momentum of the moving wagons. The Driver's Safety Device (deadman's pedal) in the shunter was disabled by a feature that allowed the driver to leave his seat to observe the passage of the train provided that the shunter's direct air brake had been applied. Had that brake been released and the deadman's pedal allowed to operate, the train's main air brakes would have applied automatically and stopped the train. Trap points, which might have prevented the runaway from leaving the sidings and joining the branch line, had not been reset after the departure of the mainline train.