Rail Accident Investigation Branch
Summary
At around 05:45 hrs on 24 December 2021, 5 wagons of a 33-wagon intermodal freight train derailed at low speed as the train was entering the rail terminal at London Gateway port, Essex. The derailment started when wheels on a wagon in the middle part of the train suddenly lifted off the track, just before reaching the port boundary, with the other wagons becoming derailed as they passed over points within the rail terminal. While no one was injured, infrastructure damage disrupted rail freight access into and out of the port for 14 days.
The first derailed wagon was the unloaded centre wagon of a triple-wagon, which was made up of three individual wagons permanently coupled together. The wagons on the train either side of the unloaded centre wagon were all loaded, and a train brake application had recently been made when the derailment occurred. A longitudinal train dynamic simulation model, specially developed as part of the investigation, showed that compressive forces generated in the train were larger towards the middle of the train, where the triple-wagon was located, and rapidly increased to a maximum as the centre wagon approached the place where it became derailed. Data from the trains on-train data recorder showed that this was shortly after brake forces acting on the locomotive had peaked and that the trains brakes would have been starting to release.
Derailments in 2015 involving an earlier version of the type of triple-wagon involved demonstrated that triple-wagons of this design configuration were susceptible to derailment when large longitudinal compressive forces are suddenly applied. Early derailment simulation studies had confirmed this and helped inform a decision to lengthen the bar coupler on the earlier version of these wagons to improve derailment resistance. The wagon designers identified the need to make other design changes when the new version of the triple-wagon was developed for unrelated reasons. However, although the risks due to longitudinal compressive force continued to be recognised, the need to make further design improvements to address them was not deemed to be necessary. Derailment simulations undertaken in support of the investigation helped show that the new version of the triple-wagon remained similarly susceptible to derailment and confirmed that the lack of payload on the centre wagon, and possibly in-service degradation of the bogie, increased the derailment risk.
The investigation found that the risks associated with the longitudinal dynamic behaviour of long freight trains are not well understood in the rail industry and that there are limited processes, tools, and knowledge resources available to assess and manage them. This was identified as an underlying factor. The investigation has identified the design management arrangements that were adopted during the development of the new version of the triple-wagon as another underlying factor. It is possible that these arrangements limited the designers ability to understand the dynamic behaviour of the triple-wagon and did not result in the identification of critical performance requirements.
Recommendations
RAIB has made three recommendations. One is directed to VTG Rail, the owner and developer of the triple-wagon, covering the need to identify and implement any necessary design changes and operating restrictions. The second is directed to GB Railfreight, the operator of the train, covering the need to develop instructions and best practice guidance for its operations staff. The third is directed to RSSB covering the need to raise the rail industrys understanding of the derailment risks associated with longitudinal compressive forces and the management strategies that need to be introduced.
Notes to editors
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The sole purpose of RAIB investigations is to prevent future accidents and incidents and improve railway safety. RAIB does not establish blame, liability or carry out prosecutions.
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RAIB operates, as far as possible, in an open and transparent manner. While our investigations are completely independent of the railway industry, we do maintain close liaison with railway companies and if we discover matters that may affect the safety of the railway, we make sure that information about them is circulated to the right people as soon as possible, and certainly long before publication of our final report.
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