At 05:50 on 20 December 1984, the train, carrying more than () of four-star petrol in thirteen tankers, entered the tunnel on the Yorkshire (north) side traveling at . One-third of the way through the tunnel, a defective axle bearing (journal bearing) derailed the fourth tanker, which caused the derailment of those behind. Only the locomotive and the first three tankers remained on the rails. One of the derailed tankers fell on its side and began to leak petrol into the tunnel. Vapour from the leaking petrol was probably ignited by the damaged axle box.
