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 949, fell into the Whangaehu River. The bridge over the river had been badly damaged just minutes earlier by a lahar from Mount Ruapehu.

Awards and recognition
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.

Warning system
Railway accidents involving bridge washaways include:
Generalising the problem of detecting bridge washaways to washaways and landslide in general, detection systems can never be foolproof or 100%.

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.
1993 - 114 perished in a passenger train that plunged into a river after floods washed away a bridge at Ngai Ndethya.
- Veligonda train disaster
See List of rail accidents for more details. See also

NZ History