Bearing behind Mercer derailment
An investigation has found that a faulty bearing on a KiwiRail freight train went undetected and caused four wagons to topple off the tracks onto State Highway 1 at Mercer.
The southbound lane was closed after four wagons derailed on a north-bound train, with two obstructing the motorway, about 3.20am on September 3 last year.
Traffic was diverted for more than 11 hours.
Two railway lines were also forced out of action. The first was reopened at 2.30pm and the second was in use by 6.30pm.
KiwiRail investigated the derailment and found a faulty wheel bearing was to blame.
The derailment involved a container train - one locomotive and 36 wagons - operating between Tauranga and Auckland.
Senior communications adviser Jenni Austin said the investigation found the cause to be a bearing failure on a wheel on one of the wagons on the train.
"We have acoustic bearing detectors set up at three locations around the network," Austin said.
"This is sophisticated electronic equipment with a series of microphones which record the sound of trains as they pass - failing bearings make a distinctive noise so those wagons where bearings are beginning to fail can be identified."
But in this case, Austin said, noise from other components on the wheel masked that sound.
"We have adapted maintenance checks to ensure those components are now tightened."
She said KiwiRail has an ongoing programme of maintenance for all wagons in the fleet with full inspections carried out every two years.
There was minimal track damage as a result of the derailment.
The long-term trend for derailments is decreasing - between 2009 and 2012 the derailment frequency was halved from about four a month down to two. It remains at that level.
Austin said that has been achieved though significant investment in rail network infrastructure and the introduction of new rolling stock.
The Transport Accident Investigation Commission (TAIC) is also investigating the incident. Their report is not due to be made public until October.
TAIC is a standing Commission of Inquiry and an independent Crown entity.
Its main purpose is to determine the circumstances and causes of accidents and incidents with a view to avoiding similar occurrences in future, rather than to ascribe blame.
It looks into significant aviation, rail and marine accidents and incidents. Usually, the TAIC does not investigate road events except when the circumstances may have significant implications - for rail safety, for example.