At first, guard Wray recommended that the train be allowed to roll back on the falling gradient to Welwyn, but the driver refused, as such a move would be dangerous and contrary to the rules.
According to regulations, Wray should have placed detonators on the line to protect the rear of his train, but he did not, and he also failed to communicate with either signal box.
The signalman at Welwyn, who had not received the "out of section" signal for the train of coal empties, sent a telegraph message to Knebworth asking if it had cleared the tunnel.
Before any of the railwaymen could communicate with either signal box, the up meat train was allowed into the tunnel, where it struck the wreckage from the first collision and caught fire.
According to Rolt, "all that night and all through the next day the ventilation shaft belched flames, smoke and the smell of roasting meat over the surrounding countryside.