The Tangiwai disaster on 24 December 1953 was the worst rail accident in New Zealand. An 11-carriage overnight express from Wellington to Auckland fell into the Whangaehu River at Tangiwai, ten kilometres (six miles) west of Waiouru. The bridge carrying the North Island Main Trunk Railway over the river had been badly damaged just minutes earlier by a lahar from Mount Ruapehu. The KA class steam locomotive, all five second-class carriages, and the leading first-class carriage derailed, resulting in the deaths of 151 of the 285 people aboard the train. Of the 176 second-class passengers aboard, only 28 survived.151 died in total
The damage inflicted by the lahar washed away one complete span and left only the rails, supported by the remaining concrete piers. When the train ran onto the bridge the rails were incapable of supporting its weight and buckled in the middle. The locomotive and first carriage were launched into the air by striking a remaining concrete pier and reached the opposite bank of the river. The impact of the accident caused the locomotive's tender to flip over the locomotive and rip the cab away from the engine, thereby killing the crew. Following the first carriage, the second to fifth carriages tumbled into the river and were torn apart, with substantial loss of life. One of the carriages was carried five miles downstream by the lahar. All five of these carriages were second-class carriages; the leading first-class carriage was sixth in the train and it teetered on the edge of the bridge before its coupling to the rest of the train snapped and, with nothing left to restrain it, it rolled into the river. The remaining three first-class carriages, the guard's van, and a travelling post office van remained on the track.
The death toll of 151 consisted of 148 second-class passengers, one first-class passenger, the driver Charles Parker, and the fireman Lance Redman. Twenty of the bodies were never found and are presumed to have been carried 120 km downriver to the ocean.
Among the dead was Nerissa Love, the fiancee of cricketer Bob Blair, who was playing in a Test Match in South Africa at the time. On going out to bat after his loss, he received a standing ovation.
The actions of numerous individuals saved many lives, with Cyril Ellis in particular credited as a hero. Ellis was driving north with his wife and mother-in-law to visit his parents for Christmas when he discovered the road bridge near the railway line was flooded and impassable. He noticed the approaching light of the train, and assuming that the railway bridge would be similarly impassable, he ran to warn the train, brandishing a torch. He stated that he leapt the railway fence, climbed up the embankment, and ran down the middle of the line towards the oncoming train while waving the torch before jumping clear. A commission of inquiry after the accident determined that the locomotive crew were aware of the danger at some point before the accident, as the driver had shut off the steam regulator valve and applied the emergency brakes, and the fireman had sanded the track for 700 metres. However, this took place too close to the bridge to avoid disaster and the commission was unable to ascertain whether Ellis's actions motivated those of the crew.
After the train crashed, Ellis informed the train's guard, William Inglis, of what had happened and the two entered the sixth carriage, then still balanced precariously on the bridge's edge, in an attempt to save passengers. While they were in the carriage, it tumbled off the bridge and Ellis and Inglis, with the assistance of passenger John Holman, smashed a window and helped passengers out of the carriage. Of the carriage's 24 occupants, only one died, a girl who was trapped in her seat and drowned.
For their actions, Ellis and Holman received the George Medal. Inglis and a passing traveller, Arthur Dewar Bell, both received the British Empire Medal for actions that saved 15 lives. Queen Elizabeth II and Prince Philip were visiting New Zealand on their first royal tour when the disaster occurred. The Queen made her Christmas broadcast from Auckland, finishing with a message of sympathy to the people of New Zealand. Prince Philip attended a state funeral for many of the victims.
Evidence given at the commission of inquiry into the disaster revealed that the midstream piers of the bridge had been undermined by previous sudden floods, from as early as 1925. Concrete blocks weighing several tons had been placed around the footings of these piers and the space between the blocks and the piers backfilled with gravel, but the lahar was strong enough to sweep these away.
The cause of the lahar was the collapse of a natural volcanic ash dam blocking the outlet of Mount Ruapehu’s crater lake. Until this disaster, the danger posed by lahars from Mount Ruapehu was appreciated by only a few scientists.
Following the disaster, the Railways Department installed a lahar warning system upstream in the river to alert train control to high river flows. Signalling equipment has also since been substantially modernised using track circuits, which warn train control of broken sections of track. This latter system does not guarantee detecting a washout, as unsupported rails may still be unbroken.
Railway accidents involving bridge washaways include: