The railway line at this point, known as the Cowan Bank, is 2.5% (1 in 40) grade and, as the special hauled by steam locomotive 3801 passed southwards through the No.
The driver of 3801 applied sand to the rails to improve traction, but the train came to a complete stop approximately 50 metres clear of the tunnel.
4 tunnel for 10 minutes by a red stop signal, received a green indication and proceeded, colliding with the steam train shortly after.
The force of the crash completely destroyed the last carriage of the steam train, and was enough to break the locomotive coupling and push the engine 12 metres forward.
Mr Hand also considered it likely that a passenger on 3801 had applied a handbrake on the third carriage, and that this, combined with the heavy load, the steep grade and the curvature of the line, had prevented the steam train from restarting.