The primary cause of the crash was incorrect wiring work on the signalling system; a redundant wire was left connected at one end, and bare at the other. The wire came into contact with a relay, causing a signal to display a 'wrong side' green aspect regardless of the presence of a train on the track circuit. The signalling technician responsible had also worked a seven day week for the previous thirteen weeks.
Twenty five years on from the Clapham Junction rail disaster, a survivor is still remembering fresh details from that harrowing day.
See the news item here, and the accident report on the Railways Archive website here.