Tangiwai disaster

The Tangiwai disaster on December 24 1953 was the worst rail accident in New Zealand. The overnight North Island Main Trunk express train from Wellington to Auckland, hauled by steam locomotive K^A 949, fell into the Whangaehu River. The bridge over the river had been badly damaged just minutes earlier by a lahar from Mount Ruapehu.

One Cyril Ellis is credited as being a hero of the accident because his actions saved many lives. He saw that the railway bridge was damaged and ran down the track, waving his torch at the oncoming train. It is believed that his warning enabled the train driver to brake before the bridge, slowing the train sufficiently so that only the first six carriages tumbled into the river. Almost everyone in the sixth carriage survived. All following carriages stopped safely.

Scale

Of the 285 people on the train, 134 survived and 151 died. Of those that died 20 bodies were never recovered; it is believed they were washed 120 kilometres from the bridge and then down the river and out to sea.

Inquiry

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.

Cause

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.

Prevention

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.

Similar accidents

Railway accidents involving bridge washaways include:

• Image:US flag 48 stars.svg September 27 1923 – near Glenrock, Wyoming - a bridge over Coal Creek was washed away and a passenger train derailed, killing 30 of the train's 66 passengers.

• Image:Flag of Kenya.svg 1993 - 114 perished in a passenger train that plunged into a river after floods washed away a bridge at Ngai Ndethya.

• Image:Flag of India.svg - Veligonda train disaster

• See List of rail accidents for more details.

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