The 08:44 from Glasgow Queen Street to Dundee was running late that morning, and not running well. Its locomotive, British Rail Class 25 number 25083, had a brake binding on the leading bogie. The driver pressed on - Dundee was only a few miles away. But as the train ran along Invergowrie Bay, a traction motor caught fire and the five-coach service was forced to stop on the main line. The crew began to manage the emergency. They expected protection from the signalling system behind them. Approximately ten minutes after they came to a halt, the 09:35 Glasgow-to-Aberdeen express, seven coaches hauled by Class 47 locomotive 47208, struck them at around 60 mph.

The impact tore the last four coaches of the Dundee train over the sea wall. The two rearmost coaches broke away completely and ended up in the Firth of Tay; the tide was out, which may have saved more lives. Both passengers in the rear carriage were killed instantly. So were the driver and secondman of the Aberdeen express. A further passenger died later. Robert Duncan was fifty-two, a Tayport man, a church elder and special constable. He had worked for British Rail since he was sixteen years old. He left a widow and a nineteen-year-old son. William Hume, twenty, lived in Fintry in Dundee; he had been with British Rail for four months when he climbed into the cab that morning. The other immediate fatalities were Dr James Preston, a community health officer aged sixty-five, and Kazimierz Jedrzejczyk, a Polish marine engineer working in Scotland far from home. The footballer Dougie Wilkie was among the seriously injured and was left paralysed from the waist down. Behind each name was a life, a family, and a morning that ended very differently from the one those families expected.

What had gone wrong at Longforgan signal box, just behind the stationary train? The signalman insisted he had put the starting signal correctly back to Danger after the Dundee train passed. When the Aberdeen express arrived a few minutes later, he cleared the Home signal for it. The train moved slowly forward, then accelerated and passed the Starting signal - which the signalman believed was still at Danger. When he went down to the track afterwards, he found the signal arm raised about four degrees. Subsequent inspection showed it could have been raised as much as eight. The guard of the Aberdeen train, looking back from the rear coach, described it as a "poor off" - somewhere between on and off - estimating an angle of 7.4 degrees. He assumed the signal had simply not yet returned fully to horizontal after being put back to Danger. The public inquiry found him not to blame.

Why William Hume passed the signal remained, in the inquiry's careful language, a mystery. He may have been looking back toward the signal box, or checking that the train was clear of the level crossing, and only glanced up at the signal at a poor angle. From close below it, with the signal arm visibly out of true, he may have concluded that it had moved since he last saw it - and that the signalman had now cleared his road. The signal post bracket itself was found badly bent, possibly struck by a chain hanging from a goods wagon or by a lineside maintenance machine. The signalling at Longforgan lacked basic safety features that became standard afterwards: no Automatic Warning System to alert a driver of a signal at Danger, no adjuster on the pull wire, no repeater in the signal box, no provision for placing detonators on the track to warn an approaching train. The Invergowrie accident contributed directly to investment in such safeguards across the network. It cost five lives to make that case undeniable.