The express was running at about 75 miles per hour along the Down Fast line through the flat countryside east of Conington, in what was then Huntingdonshire. D9004 was a Deltic, one of the most powerful diesel-electric locomotives the East Coast Main Line ever ran. The Queens Own Highlander, like its sister Deltics, was famous for its bellowing twin Napier engines and for the speed at which it could move a heavy train. On the night of the accident, the train was loaded with passengers travelling north overnight, some of them asleep. The driver and his secondman saw the Home signal at Connington South displaying Green as they approached. They were still watching it as it passed out of their line of sight. Behind them, the rear of the train was about to be derailed.

The Railway Inspectorate investigation, led by Lieutenant Colonel I. K. A. McNaughton, found no fault with the track, the train, or the signalling equipment itself. The interlocking at Connington South recorded the Home signal as having been at Danger when the points moved. The crew said it had been Green. Both could be true. The investigators worked out a sequence that fit every piece of evidence: just as the locomotive reached the Home signal, the signalman replaced it to Danger, pulled the point lock lever to withdraw the mechanical lock, and lifted the point lever latch before the electrical track-circuit lock could engage. The whole sequence had to occur in under two seconds, in the gap between the train passing the signal and reaching the track circuit just beyond it. Tests on a similar frame showed that an experienced signalman could just about do it. The interlocking, the heart of British railway safety, could be defeated by someone who knew exactly how it worked and was willing to try.

Alan Frost was twenty years old. He had joined British Railways in January 1965 after being discharged from the Royal Marines with a recorded diagnosis described in the medical paperwork as hysteria and immature personality. The railway management did not know this, although his references had been taken up. After the derailment, he told investigators he had been swinging on the levers and had accidentally changed the points. The investigation found that account impossible. To produce the derailment the lever had to be held slightly out of its frame for some seconds, moved just as the sixth coach was passing over the points, and then returned to its normal position. That was not an accident. It was a sequence of deliberate movements. Frost never offered an explanation for what he had done. He is not recorded as having said anything at all about the five deaths he caused.

The Connington South report is one of the strangest documents in the history of British railway safety because it explains exactly how the accident happened and refuses, finally, to say why. The signalman's earlier diagnosis was disclosed too late to matter to the railway, and the inquiry recommended better employment screening for safety-critical positions, particularly the sharing of military medical histories. The investigation also led to changes in how interlocking frames were designed so the precise defeat sequence Frost used would be physically harder to execute. But none of that touches the central question. A man stood at his lever frame on a Sunday night, watched a sleeper train approach, and made a sequence of deliberate movements that he knew would derail it. He did not run. He stayed at his post. He gave a false explanation that the evidence collapsed. And he never said what he was thinking.

The crash happened in a stretch of countryside that, even now, is some of the emptiest in southern England. The East Coast Main Line runs dead straight here across reclaimed fenland west of Whittlesey, past isolated farms and drainage ditches. At night the only lights for miles are the signal boxes and the trains themselves. Five people died on a Sunday night in a place no one was thinking about, on a journey they expected to sleep through. The wreckage lay across the lines until the following morning. There is no memorial. The signal box at Connington South has long since been demolished, replaced by remote-controlled signalling from a panel many miles away. The names of those five passengers are recorded in the inquiry papers and in a few newspaper reports from March 1967. They were on their way home, or on their way to start something, and they did not arrive.