The Tangiwa disaster was New Zealand’s worst rail accident, which at face value looks out of place on this site. but the incident was infact indirectly caused by the earlier eruption of Mount Ruapehu.
Mount Ruapehu sits at the southern end of the Taupo Volcanic Zone and is the largest active volcano in New Zealand. In 1945 eruptions began in March and ran intermittently through out the year. Activity varied greatly from gentle steam plumes to doming in Crater Lake.
The activity in 1945 had several dire consequences. In July geologists Robin Oliver and J. Witten Hannag has a lucky escape when an explosion showered them with hot rocks and ash leaving Oliver unconscious and seriously burnt. The heavy ashfall through out the last few months of the year also led to the closure of a hospital 9 km away from the crater as ash kept penetrating the generators. Over the year hundreds of people were diagnosed with what doctors termed ‘Ruapehu’s Throat’ where people were suffering with breathing difficulties from inhaling the dense ash. But the worst of all was the Tangiwa disaster.
The activity of 1945 carved out the crater deeper than it previously was and once activity had ceased the crater began filling with water. By 1953 the water level had risen over 8 meters higher then it was before the eruptions and was only contained by an unstable mass of ice and volcanic rubble and ash. At 8 p.m. on Christmas Eve 1953, the debris at the outlet of Crater Lake collapsed sending 340,000 cubic metres of water pouring into the head of the Whangaehu River. It swept down the valley, picking up sand, silt and boulders as it went. Soon after 10 p.m. the lahar smashed into the main trunk railway bridge at Tangiwai. The concrete piers were knocked out and the bridge partially collapsed.
A passenger train from Wellington, packed with 285 people heading to Auckland for the holidays had no idea what lay ahead as it approached the bridge in the dark. A local who saw the bridge collapse tried to flag down the speeding train but even though the driver saw him and applied the brakes the train was going to fast and still carried on to the bridge. The engine and first carriage nosedived, landing against the opposite bank. Four more carriages plunged into the river, floating in the torrent briefly before sinking. Another four carriages remained on the track, but one of them dangled over the river.
One carriage was carried more than 2 kilometres downstream. The others were swept across the flooded main road or rammed into the riverbanks. Some people had escaped and swam to the banks, but dozens drowned in the tangles of gorse there. The work of recovering victims went on for several days along 60 kilometres of the river. Twenty bodies were never found; it was assumed they had washed out to sea, some 120 kilometres away. 151 people lost there lives due events set in motion by a volcano 8 years earlier.