A major rail derailment near Palmerston was caused by a failed bearing, it has been revealed.
A northbound freight train hauling 20 wagons laden with meat and byproducts derailed near Palmerston on February 19.
That derailment forced the closure of the main south railway line and sparked an investigation.
KiwiRail this week released details of the derailment, which was caused by ''a failed bearing on the wheel of one wagon'', a spokeswoman said.
Initially, only one or two wheels derailed and the wagon remained upright. The train continued as normal for a considerable distance, before the damaged wagon and eight others tipped and left the track, she said.
She confirmed KiwiRail had a testing regime for monitoring the condition of wheel bearings.
Acoustic detectors placed around the network recorded the sound of wagons as they passed. A failed bearing emitted a specific noise enabling it to be identified and repaired.
''In this instance, the bearing failed very quickly - in a space of only one to two days. This is a rare occurrence and quite hard to guard against,'' she said.
The driver, the only person on the train at the time of the derailment, was uninjured. Repairs after the crash cost $800,000.
KiwiRail said preliminary investigations suggested failings with a New Zealand-made wagon. However, the spokeswoman confirmed the wagons were made in Korea in the 1970s.
