Lightly-loaded, high profile wagons combined with an unfavourable coupling setup, and what was most likely an extreme weather event, to derail an SCT Logistics train in Western Australia late last year, the Australian Transport Safety Bureau has found.
At around 2:50pm on December 4, 2015, 39 wagons derailed from SCT’s 59-vehicle 5PM9 service on the interstate railway near Rawlinna, WA.
A fire started within one of the wagons, a refrigerated food van, and later burned itself out. There were no dangerous goods involved, the Bureau said, and no fuel spillage was reported.
No physical injuries were reported, but two relief drivers were said to have been badly shaken when the crew van rolled on its side.
The derailment also damaged around 550 metres of track.
The ATSB’s report, released last week, found the configuration of multiple SCT PBHY ‘high cube’ wagons “provided a large side surface area that was susceptible to high wind forces”.
The train was driving through a thunderstorm when the derailment occurred, but the Bureau believes conditions were quickly worsened by a ‘microburst’; an intense local weather condition which is difficult to predict or detect on meteorological charts.
Based on reports from the train’s drivers, the first wagons to roll over in the high wind conditions were the lighter, unloaded PBHY wagons, which are 21.65 metres long, and rise to 5.59 metres above rail level, giving them a side surface area of 105 square metres.
When the lighter wagons rolled, they took several heavier, loaded wagons with them, due to the nature of the coupler setup on the PBHY wagons, the Bureau believes.
The PBHY wagons involved in the incident had a bottom retention shelf, designed to stop a mating coupler from falling to the ground, in the event it had separated from its wagon, the Bureau reported.
“The intent [of the bottom retention shelf] is to reduce the risk of derailment of the following vehicles due to a dislodged coupler passing beneath a wagon,” the ATSB said.
“However, in the context of this derailment, the bottom shelf coupler ensured the adjacent wagon remained coupled during the rollover sequence. This transferred the rotating force between wagons via the coupler.
“Consequently, the loaded wagons which may have been less susceptible to rollover from wind induced force, also had rotating force applied through the couplers.”
The Bureau found three derailments in the past with similar factors: A 2006 derailment near Tarcoola, South Australia, a 2008 derailment on the Nullabor Plain in WA, and a 2010 derailment on the Central-Australia Railway line in South Australia.
All three investigations aligned with the 2015 Rawlinna derailment, in that a combination of higher-than-predicted wind speeds due to a suspected microburst, and lightly loaded or empty freight vehicles with large side profiles, led to a derailment.
As a result of the 2015 derailment, SCT Logistics issued a rail safety alert to all operations staff advising drivers to slow trains to below 70km/h when experiencing severe weather conditions involving strong cross winds.
The company is also reviewing and amending its Risk Assessment Register – Mainline Operations to include elements of risk to operations due to adverse weather conditions.
The ATSB said the takeaway message from its investigation was that operators should consider the effects of severe weather conditions, and how this can affect the safety of train operations.
“Within the geographical regions of their operations, operators should implement risk management strategies to minimise the likelihood of train derailment when severe weather conditions are forecast,” the Bureau concluded.