The 22:30 express from King's Cross to Edinburgh, hauled by a Deltic locomotive, was travelling along the Down Fast line at around 75 mph when the rear portion of the train was derailed to the left. The last four coaches came to rest on their sides and two others were derailed.
No fault was found with either the track or the train.
Immediately after the accident, the signalman claimed that he had accidentally changed the points while "swinging" on the levers. The public inquiry into the accident surmised that, as the train approached Connington South signal box, the signalman had:
This sequence would have had to have occurred in the time between the train passing the Home signal and running on to the track circuit, i.e. in less than two seconds. Tests were conducted using a similar signalling frame to the one at Connington South and it was found that an experienced signalman could just about manage to reproduce the sequence. Thus, it was shown that the interlocking could be defeated.
It was clear that the signalman had stood for some seconds with the points lever slightly out of its frame, moved it just as the sixth coach was passing over it, then returned it to its normal position. This would be a premeditated rather than an accidental act.
The signalman had entered the railway service in January 1965 after serving with the Royal Marines. He had been discharged after suffering from "hysteria and immature personality", but this was not known to the railway management at the time, even though his references had been taken up.
He was tried on charges of manslaughter and endangering the safety of railway passengers in November 1968. After a trial lasting 11 days, the judge instructed the jury to acquit him on the charges of manslaughter and sentenced him to two years' imprisonment for unlawfully operating the signal and points mechanism of the Connington South signal box so as to endanger persons being conveyed on a railway, on which charge the signalman had changed his plea to guilty.