Executive summary
Between March 2007 and October 2009 there were 5 derailments involving container wagons conveying single 6-metre (m) containers loaded with bulk grain, positioned on the leading ends of the wagons. The Transport Accident Investigation Commission (the Commission) combined the events into a single inquiry.
In 2005 the Commission made a recommendation to review the current track geometry standards and the wagon maintenance standards to ensure those standards are compatible and minimise the potential for derailments caused by dynamic interaction, where the condition of the wagon and the speed at which it is travelling interact with one or more rail track faults. This can cause the wagon to oscillate and derail (see Appendix 7 for a description of dynamic interaction).
For most of the period covered by these accidents the freight train operations and track maintenance and management were with different entities. Today, responsibility for both rests with KiwiRail Limited.
In 2009 KiwiRail refined the way it measured and managed track faults and prioritised their repair. In 2011 it tightened wagon wheel maintenance requirements. These improvements reduced the risk of dynamic interaction-related derailments. In 2010 KiwiRail installed acoustic bearing monitoring equipment at 3 key points on the rail network to detect failing wheel bearings before they collapsed; another cause of derailments that had resulted in a Commission recommendation. Mainline derailments have about halved during KiwiRail's tenure.
The Commission believes the actions taken by KiwiRail have significantly reduced the likelihood of future occurrences similar to those that led to this inquiry.
The key lessons learnt from the inquiry into these occurrences were:
- sufficient resources need to be put into maintaining rail track infrastructure and rolling stock if mainline derailments are to be kept as low as reasonably practicable
- if the allowable tolerances for the condition of rail track and rail wagons are not compatible, dynamic interaction between the track and wagons will result in an increase in mainline derailments.