On 8 October 1952, at around 8:17 a.m., the local train stopped at platform 4 at Harrow and Wealdstone station, approximately seven minutes late because of fog. Carrying about 800 passengers, it was busier than usual because the next Tring-Euston service had been cancelled. As scheduled, it had travelled from Tring on the slow line, switching to the up fast line just before Harrow and Wealdstone to keep the slow lines to the south of the station clear for empty stock movements. At 8:19 a.m, just as the guard was walking back to his brake van after checking doors on the last two carriages, the Perth express crashed into the rear of the local at a speed of . It had passed a colour light signal at caution and two semaphore signals at danger, and had burst through the trailing points of the crossover from the slow lines, which were still set for the local train.