King's Cross station drops away from its platforms into Gasworks Tunnel - a three-bore Victorian tunnel that runs north under Regent's Canal and out toward the East Coast Main Line. The track dips for the first 146 yards, then climbs again at a gradient of about 1 in 105 for a mile and a quarter. For heavy trains the gradient was always tricky, particularly when starting cold from the platform with steam still building. Since December 1943 it had become normal practice for the locomotive that had pushed the empty coaches into the platform to remain coupled at the rear, helping to shove the train through the first hundred yards of climb. On the morning of 4 February, the worn rails on the No. 1 down main line had been replaced as part of routine maintenance. The new rails had a smoother, less adhesive surface. Earlier in the day, the first empty train to try the climb had slipped to a stand.

The 18:00 to Leeds was formed of seventeen coaches behind a streamlined Class A4 Pacific - locomotive number 2512, Silver Fox. The A4 was the same class as Mallard, the world steam speed record holder, and Silver Fox had been working East Coast expresses since 1935. On the wartime evening of 4 February she was in good general condition, except for the sanding gear, which was meant to drop sand onto the rail under the driving wheels for extra grip. It had been giving trouble all day. The coaches had been propelled into the platform rather than hauled, which meant there was no banker engine coupled to the rear - the assisting locomotive that should have shoved the train up the first hundred yards. The train left platform 5 five minutes late, in the dark, with seventeen coaches behind a single struggling Pacific and a tunnel of greasy new rail ahead.

Inside Gasworks Tunnel, on the climb, Silver Fox lost her grip. The driver fed steam but the wheels spun on the new rail; without sand the locomotive could not bite, and with no banker behind there was nothing to push. The seventeen coaches gradually slowed, then stopped, then - silently and at first imperceptibly - began to slip backwards down the gradient. The driver, working blind in the dark tunnel and concentrating on the regulator, did not realise his train was moving the wrong way. The signalman on the box at platforms 5 and 6 noticed before the driver did. He saw the rear of the train approaching back through the tunnel mouth and reset points 145 to send it harmlessly into empty platform 15. But he was a fraction of a second too late. The first bogie of the rear coach had already crossed the points. The second bogie went into platform 15. The two halves of the same coach took different tracks.

The rear coach was a Vestibuled Brake Composite, number 1889, built at Doncaster in 1941 - a 61-foot teak-bodied carriage with two first-class compartments in the centre, three third-class compartments to one side, and the guard's brake to the other. When the two bogies pulled in opposite directions, the coach twisted and lifted, rose into the air, and struck the signal gantry above platform 10, where the 19:00 Aberdonian was already loading. The gantry crushed the centre of the carriage - the precise section where the two first-class compartments were. Twenty-five passengers were injured. The train attendant was hurt. Two passengers died: a railway labourer and Cecil Kimber, sitting where the first-class fares had placed him. Kimber's death was the kind that newspapers noticed - the man whose initials still ride on a few thousand sports cars - but the accident's true peculiarity was that no one had ever quite imagined a train could end this way.

Coach 1889 was so badly damaged that it was written off. It had been scheduled to be renumbered 10153, but the new number was simply never used. The signal gantry was rebuilt two weeks later; full services resumed on 23 February. The Inspecting Officer of Railways, Colonel Wilson, wrote a measured report that placed the main fault on the driver - he ought to have realised he might roll back after such prolonged slipping, and he did not realise for some minutes after stopping that a collision had even occurred. Wilson's report also noted the contributing causes: the faulty sanding gear, the new rail, the absence of a banker, the night. Successive inspectors went on to call the accident 'somewhat bizarre' and 'stupid' - reviewer's shorthand for something that wasn't supposed to be able to happen but did. Modern signalling, with continuous track circuits and roll-back protection on locomotives, has made the King's Cross accident effectively impossible to repeat.