A wheel failure that led to a train derailment, damaging 590 m of track, likely originated from a crack on the tread surface, the ATSB has found.
On 28 August 2015, a Pacific National fully loaded coal train MB520 departed from Maules Creek, NSW destined for Port Waratah, NSW. Shortly after passing Ardglen and descending the grade towards Pages River, a wheel failure occurred on the leading right hand wheel on the 19th wagon. This led to the wheel moving off the wheel seat towards the centre of the axle. The bogie rotated causing the other wheels on the bogie to derail. The train continued travelling until a damaged inter car brake cable activated the train’s brakes. There were no injuries, but the derailment caused damage to over 590 metres of track including 963 sleepers.
What the ATSB found
The ATSB investigation found that the derailment was caused by a wheel failure. A metallurgical analysis conducted after the derailment found that the wheel had multiple cracks in the wheel rim; one crack extended from the rim to the boss of the wheel. This likely originated from a transverse thermal crack on the wheel tread surface. Thermal cracks of this type are generally associated with high thermal input under service brake conditions.
An increasing level of wayside alerts starting a month before the derailment indicated the likelihood of the fault being present then. Despite the increasing impact level readings, no immediate action was taken to inspect the wheel before it eventually failed.
It was found that the wheel inspection processes were not effective in detecting surface damage or cracks. The failed wheel was approaching the end of its service life with a rim thickness of 25 mm. This low rim thickness increases propensity to thermal distortion and is likely a significant factor into the thermal crack formation and propagation that occurred.
At the time of writing this report there have been seven reports of fractures detected in wheels, within a 3-year period, on Pacific National coal wagons in NSW. Three occurred before the derailment with the first incident on 31 May 2013. Three more fractured wheels were identified within a two-month period after the derailment. Following the first three wheel failures internal reports recommended changes to reduce the risk of wheel defects. Only limited action was taken by the operator prior to the derailment on 28 August 2015.
Since the derailment, the rollingstock operator has advised that they have established a process to remove wheels considered to be at greater risk of fracture. To this end, they have implemented a program to remove wheels with a rim thickness under 25 mm. Pacific National also intends to improve their internal standards to include revised criteria and actions for thin rims and wheel impacts.
Safety message
Rollingstock operators with heavy haul wagons using wheels near the end of their service life should be aware of the increased risk of wheel failure due to cracking. They should ensure that wheel inspection and maintenance programs include systems and techniques for detecting and assessing wheel defects with the potential to lead to cracking. These systems and techniques should be validated to ensure they are effective to detect such defects.
Read the report: Derailment of freight train MB520, Pangela, New South Wales, on 28 August 2015