[2] Because of the gradient in the tunnel, it had been the practice since December 1943 for heavy trains to be assisted for the first 100 yards (91 m) by being propelled by the locomotive which had hauled the empty coaches into the platform.

[4] The newly laid rails had lower adhesion, and the first (empty) train to travel on them slipped to a stand on the incline.

[9] The design, known as Diagram 314, used a steel underframe 60 feet (18 m) long, mounted on two bogies each having a wheelbase of 8 ft 6 in (2.59 m), spaced at 43-foot (13 m) centres.

The body was 61 feet 6 inches (18.75 m) long, 9 ft 3 in (2.82 m) wide, and built largely of wood, principally teak.

[12] On this occasion the train was not assisted, because the coaches had been propelled, rather than hauled, into the platform, and so there was no locomotive at the rear as was the usual arrangement.

[3] The train left platform 5 at King's Cross station five minutes late, and entered Gasworks Tunnel.

[13] When it reached the rising gradient at the far end of the tunnel the locomotive began to slip badly on a section of newly replaced rail.

Preoccupied with his tasks at the controls and operating in darkness the driver didn't notice when the train slowed to a stop, and then began to run backwards.

[14] The signalman became aware of the 18:00 train rolling back and operated the points again in order to route it into unoccupied platform 15, but he was too late; the first bogie of the rear coach (BCK no.

In an emergency measure hand signallers were introduced to control main line trains using platforms 6 to 17, as well as movements to and from the locomotive yard.

[4] A similar accident occurred at Glasgow Queen Street in 1928, involving a lighter train but on a much steeper gradient.