The Ramsgate train struck the back of the Hayes train at 6:20 p.m. The eighth coach of the Hayes service was telescoped by the ninth and destroyed. The tender and leading carriage of the Ramsgate train derailed and slammed into a pier of the overhead bridge. The bridge collapsed, dropping its girders onto the two carriages below. Two minutes after the impact, a third train approached on the bridge and stopped just short of the gap, its leading coach tilted into space but holding. The death toll was 90. Another 173 were injured, of whom 109 were admitted to hospital. To this day the Lewisham crash remains the third-worst railway accident in British history, behind only Harrow & Wealdstone in 1952 and Quintinshill in 1915.

The names of the dead at Lewisham have never become as familiar as those of larger disasters, but they were ordinary commuters. Office workers heading home to Bromley and Beckenham. Shop clerks who had stayed an hour late to catch the bus to a Christmas show. Schoolchildren who had been to a matinee. Wives meeting husbands at suburban stations. Henry Chadwick, a man living near the railway line, ran out of his house when he heard the impact and spent hours helping pull survivors from the wreckage of telescoped carriages. He later sued British Railways for the psychiatric injury he suffered that night, a case that became a 30-year legal precedent for compensating bystanders in disasters. The Salvation Army, the Women's Voluntary Service, and St John Ambulance arrived alongside the fire brigade and the police. By 10:30 p.m., every injured person had been moved to hospital. Some had survived to retell the story for sixty years afterward; others died within hours.

The driver of the Ramsgate train, William Trew, was charged with manslaughter. He had passed two caution signals without slowing, and applied the brakes only after his fireman saw the danger signal and called out. The cab of the Battle of Britain class was known for poor sightlines; the Ministry of Transport report would later recommend wider windscreens. The coroner's jury declared the deaths due to gross negligence, but the coroner rejected the verdict and substituted accidental death. Trew was tried for manslaughter twice. The first jury could not agree. The second acquitted him. There was no clean villain to put at the centre of the story. The Ministry of Transport report, published in 1958, concluded that an Automatic Warning System would have prevented the collision. Installation of AWS had been agreed after Harrow & Wealdstone five years earlier, but priority had gone to main-line semaphore routes. Lewisham was working on coloured-light signalling, and the system had not yet reached it.

All four tracks under the bridge and the two over it were blocked. The wreckage took six days to clear. The track was relaid; the lines under the bridge reopened at 5:00 a.m. on 12 December. The bridge itself was a different problem. It was replaced by a temporary military trestle structure, the kind built in wartime to span a destroyed crossing, and that trestle is still there. Sixty-eight years later, trains continue to run across it daily. There is a plaque at Lewisham railway station, set in the wall near the main entrance, listing the date and the loss of life. Most commuters pass it without noticing. The St Johns local history group has done research into the names of every passenger who died, but the official memorial does not list them, and the city has never built a larger one.

The crash hastened the rollout of Automatic Warning System across British Rail in the 1960s and 1970s. Fog-related collisions, which had been a regular feature of the British railway from its earliest decades, declined steeply once the new system warned drivers of cautions and danger signals by audible tone in the cab. By the time the older steam locomotives were retired in the late 1960s, Lewisham-style accidents had become almost unthinkable. The third train on the bridge that night, the one that braked just in time, was carrying its own load of nearly a thousand commuters. If it had not stopped, the casualty figure would have been almost twice as high. South London still runs through this junction every minute of every weekday, and the fog still rolls in over the Ravensbourne river some winter evenings. The trains slow now. The Lewisham dead bought that slowness, in 1957.