Mount Ruapehu, at 2,797 metres high, is the highest point on New Zealand’s North Island.
One of the largest active volcanoes in New Zealand, it is part of Tongariro National Park.
On Christmas Eve 1953, a lahar, or flow of rock, mud, water and other volcanic debris, swept down the valley towards the railway bridge over the Whangaehu River at Tangiwai.
The lahar swept away the fourth pier of the railway bridge and damaged the fifth, creating a yawning gap in the bridge just ahead of the Wellington – Auckland express train.
At 10:21pm on 24 December 1953, the locomotive and the first six carriages plummeted through the gap in the bridge and into the raging lahar torrent below.
The force of the torrent carried one carriage 8 kilometres, while some bodies were found 50 kilometres away.
Of the 285 people on board, 151 were killed in the tragedy.
More would have been killed but for the actions of the postmaster from Taihape, a town situated 10km away, who saw the damage to the bridge and attempted to warn the approaching train by running along the track waving a torch.
An inquest found that the driver was able to apply the brakes before reaching the bridge, which no doubt prevented the entire train from plunging into the lahar torrent.