At approximately 0530 on 23 October 2011 Pacific National freight train 7SP5 derailed near Wirrinya, New South Wales. Train 7SP5 was travelling from Stockinbingal towards Parkes and was 14 hours into its journey from Sydney to Perth.
The lead bogie of the 42nd wagon of the train derailed three kilometres north of Caragabal. The wheels of the bogie ran derailed for about 15 km until reaching the turnout at the southern end of Wirrinya where wagons separated from the train and overturned.
The train crew were not injured. The last six wagons (all of which were multiple platform type) derailed. The derailment caused significant damage to the track and the turnout at Wirrinya.
What the ATSB found
The ATSB found that a dip in the track with adverse twist close to Caragabal caused the bogie to derail. The dip was caused by the formation subsiding due to localised formation weakness, resulting in an inability of the formation to support the track structure above.
There was no evidence of formation damage due to re-sleepering or any pumping of formation material up though the ballast and the track geometry appeared to be relatively stable for some kilometres either side of the derailment site. Track inspections had been conducted in accordance with the NSW Base Operating Condition Standards, but it is unlikely that the inspections would have identified any warning signs of formation weakness before the derailment. This was supported by the drivers of train 7SP5 not noticing anything unusual as they traversed this location. Based on available evidence, it is likely that the track dip developed under train 7SP5 and was caused by an undetected weakness in the track formation.
There was a history of track geometry defects around this location, but they were generally not significant when compared to defects identified and rectified in other locations. Notwithstanding this, analysis of track defect history is important for planning maintenance activities. There may be an opportunity for greater examination of maintenance history and defect data to help strengthen a predictive maintenance system.
What's been done since
The ARTC has implemented their Engineering Code of Practice (CoP) for its rail network in NSW as part of an ongoing program of procedure standardisation across the ARTC rail network. The CoP is slightly more stringent than the previous standards in its assessment of identified track defects. There was no evidence of pre-derailment track defects in this case, but where the formation may begin to collapse without resulting in a derailment, it is possible that the CoP may resulted in earlier maintenance action and potentially prevent a future derailment.
The opportunity may exist for managers and maintainers of track infrastructure to strengthen their predictive track maintenance systems by considering greater examination of historical maintenance and defect data.