Rail Accident Investigation Branch
Summary
On the night between 5 and 6 March 2021, a wagon with severe wheel flats on one of its wheelsets fractured two rails within a mile of each other between Pencoed and Llanharan. The wagon was part of train 6A11 which was travelling from Robeston oil terminal, Milford Haven, to Theale oil terminal, near Reading.
The wheel flats occurred because a wheelset had stopped rotating (locked) while the train was moving during the journey. The investigation found that the wheelset had probably locked during braking in an area of very low railhead adhesion, when the train was travelling along the recently reopened Swansea District line. The rails on that line were rusty as it had not been used for several months. The environmental conditions were such that the rails were also wet, and it was the combination of rust and moisture which created the very low adhesion experienced by the train.
Network Rail had not taken any specific precautions to ensure that an adequate level of adhesion was available when reopening the line. This arose because Network Rails focus when managing low adhesion was on the autumnal leaf fall season and it had not acted on the advice provided by a cross industry working group on the adhesion-?related precautions to take when reopening an unused line.
Ferryside investigation
In light of the findings of this report, RAIB considers that very low adhesion may be an alternative potential causal factor of relevance to the Ferryside accident (RAIB report 17/2018). An addendum has been added to the Ferryside report discussing this potential causal factor. This update does not alter the safety recommendations made in the earlier version of the report.
Recommendations
RAIB has made one recommendation to Network Rail to review its processes in light of the existing industry guidance to manage all occasions outside the leaf fall season which could result in very low levels of wheel/rail adhesion.
RAIB has also identified one learning point for signallers to remember that, in accordance with the Rule Book, they must arrange for a train to be stopped and examined if they become aware of an unusual noise coming from a wagon.
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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