The most likely reason for the side frame failures is that they had reached the end of their fatigue lives. A lack of identifying marks and records of side frame age and maintenance history means that it will be difficult to predict the onset of fatigue cracking in bogie side frames. The presence of fatigue cracking initiating in the region of the radius, just beyond the wear plate of the side frames, is difficult to detect by normal visual inspections, and at the current rate of MPIs the entire inventory of bogie side frames will not be completed until 2020. With 60% of the side frames tested consistently requiring weld repairs before being returned to service, the failure and collapse of a bogie side frame have resulted, and will continue to result, in significant damage to rolling stock and track infrastructure and the risk of serious harm to people.
The Commission recommends that the Chief Executive of the NZ Transport Agency give high priority to working with industry to conduct a formal assessment of the risks that main-line derailments pose to rolling stock and track infrastructure, and the risk of serious harm to people (industry and the general public). The results of the risk assessment should then be used to set appropriate remedial measures to reduce the likelihood of bogie side frame failures, and could also be used to set appropriate measures to reduce main-line derailments from other causes.
