It was caused Causation when a busy signalman , Alfred Sutton , forgot GiveUp about a pair of light engines waiting Motion at his down ( northbound ) starting Process_start signal to return Arriving to their shed at Carlisle .

They were still waiting there when the signalman set Placing the road for the down Scotch express .

The signalman was also at fault for not checking Scrutiny that the electric train had stopped Process_end before allowing Preventing_or_letting the steam train through .

Three people died Death in the crash after a signalman 's error routed Motion a small freight train into the path of an express passenger train which slowed but could not stop before colliding with it .

He applied Using the brakes but was only able to reduce Cause_change_of_position_on_a_scale speed to , before colliding with the freight train which was stationary on the same line , having stopped when the signalman showed the driver a red flag .

It was found Know that the signalman at Knowle and Dorridge signal box in the station had forgotten GiveUp he had signalled the line was clear for the express train when allowing the freight train onto the line in the path of the express .

As a contributory factor to the crash , it was found Know that the signalman , in clearing the express , had disregarded a standing regulation known as Regulation 4A , governing the safe operation of signals in areas where the minimum stopping distance for trains was shorter than the distance between signals .

However , it was not determined Deciding at the enquiry whether the signalman had cleared the express before or after allowing the conflicting freight movement .

The crash was a rear-end collision Catastrophe caused Causation by a signalman 's error .

The inexperienced signalman at Purley Oaks forgot GiveUp about a train from Haywards Heath to London Bridge standing Placing invisible in the fog .