The 3 p.m. express from Wellington carried 285 passengers and crew in 11 carriages: five second-class, four first-class, a guard's van, and a postal van. The train passed through Tangiwai station right on time at 10:20 p.m., travelling at roughly 40 miles per hour. A passerby named Cyril Ellis stood beside the tracks waving a torch, trying to warn the driver of what he could see happening at the bridge ahead. Driver Charles Parker either saw Ellis or saw the bridge itself failing. He shut off steam and applied the emergency brakes while fireman Lance Redman cut the oil supply to the fire. But the train could not stop in time. The bridge gave way, sending the locomotive and the five second-class carriages into the torrent. The lead first-class carriage teetered on the broken edge before its coupling snapped and it too rolled into the river. The remaining carriages stayed on the tracks.

Prime Minister Sidney Holland drove through the night from Auckland, arriving at Tangiwai early on Christmas morning. He coordinated rescue efforts that brought together railway workers, the New Zealand Army, police, Ministry of Works staff, local farmers, and undertakers. The army worked near the accident site while farmers searched the Whangaehu River downstream, recovering bodies at Fields Track, Mount View, Mangamahu, Kauangaroa, Whangaehu village, and the river mouth. Local people recovered 60 bodies from the Mangamahu section alone over the following days. The recovered were taken by truck to Wanganui and then by rail to Waiouru for identification. For their courage that night, Cyril Ellis and another rescuer, Holman, received the George Medal. Arthur Dewar Bell and a man named Inglis received the British Empire Medal for actions that saved fifteen lives.

Queen Elizabeth II and the Duke of Edinburgh happened to be in New Zealand on their first royal tour when the disaster struck. The Queen finished her Christmas broadcast from Auckland with a message of sympathy for the country. The Duke of Edinburgh attended a state funeral for many of the victims at Karori Cemetery on 31 December. Among the 151 who died was Nerissa Love, the fiancee of New Zealand cricketer Bob Blair, who was playing a test match against South Africa at the time. When Blair went out to bat after learning of her death, the crowd gave him a standing ovation. The intersection of national tragedy and personal grief made their story one of the most remembered aspects of the disaster, later dramatized in a 2011 television film, Tangiwai: A Love Story, and a stage play, The Second Test.

The board of inquiry, which sat from January to April 1954, found that the lahar had removed the bridge's fourth pier just minutes before the train crossed. The bridge had been designed for foreseeable flooding and for previous lahars, but the 1953 event had different characteristics and forces that no one had predicted. The board cleared every member of the train crew and every railway worker of any failure of reasonable care. In the decades since, the railway has installed progressively more sophisticated early warning systems. The current system, in place since 1999, uses radar to measure the Whangaehu's water level and transmits data to the Network Control Centre in Wellington. If levels spike, signals on either side of the Tangiwai bridge automatically switch to danger and trains are warned by radio. Since 2002, the Eastern Ruapehu Lahar Alarm and Warning System has provided backup. In March 2007, a lahar of similar magnitude to the 1953 event reached Tangiwai. The warning systems worked as designed, stopping trains and vehicles before the surge arrived. The rebuilt bridges held.