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Scientists comment on a train derailment in Sussex and a possible link to extreme heat.
Peter Dearman FREng, a Fellow of the Royal Academy of Engineering, said:
“The Lewes derailment on 13 August is under active investigation by RAIB and we must avoid speculation about its cause, but there are important lessons to learn from previous investigations. After the extreme heat events in summer 2022, Network Rail commissioned an investigation into the resilience of the railway against extreme heat headed by Sir Douglas Oakervee CBE FREng and supported by other Fellows of the Royal Academy of Engineering. That report also acknowledged the significance of the investigation carried out in the aftermath of the 2020 derailment at Carmont, led by Lord Robert Mair CBE FREng FRS, which covered the effect of climate on earthworks.”
“The 2023 Extreme Heat Taskforce report* ran to 135 pages and set out 47 recommendations covering maintenance and asset stewardship of railway track, electrification overhead lines, and critical power supply infrastructure. The report also highlighted the importance of and presented recommendations for changes and adjustments in procedures/standards. Detailed recommendations focussed on the structural organisation of and operational responsibilities/accountabilities within the engineering disciplines.”
John Lawrence, Chair of the IET Railway Technical Network, said:
“If the issue is loss of track alignment, then it could be heat expansion in rails causing the rails to expand and buckle. This stress can be compounded by multiple days and evenings of hot weather, not allowing heat to dissipate. Add to this the stresses applied to the rail during the passing of a train and this can cause a buckle under the train.
“National Rail mitigate against this by imposing speed restrictions at different temperatures measured by remote equipment on track, to the point of stopping trains when it is too hot.
“There are quite a lot of activities ongoing on the railway to research and prevent this risk.
“Another cause could be soil moisture deficit (SMD). This particularly affects areas built on clay, where prolonged dry periods can cause shrinkage of the track bed and distort the rails, which again can buckle under the train movement stress.
“National Rail mitigate against this by managing vegetation at known high risk areas, cutting down high wood and deep rooted trees and vegetation that soak up large amounts of moisture. Planting instead low moisture-dependent foliage to allow these to manage soil stability under heavy rain.”
Prof Phil Blythe, Professor of Intelligent Transport Systems, Newcastle University, said:
“While not yet confirmed, buckled rails related to the extreme heat could be a cause of the incident near Lewes.
“We are learning and Network Rail is adapting to deal with these more extreme temperatures, but not fast enough. The DARe national hub is advising government on this and building tools to help assess the risk, performance and costs of making our infrastructure more resilient. The Department for Transport’s resilience strategy should be published in the next month.
“As well as considering the impact of temperatures on the infrastructure itself, there is also concern for passengers on delayed journeys due to an incident like the derailment which can be serious due to heat and not having enough water available.”
Dr Ioannis Koliousis, Associate Professor of Logistics & Supply Chain Management, Cranfield School of Management, said:
Is heat a potential cause of derailment? If so, how will this be investigated?
“Yes, extreme heat is a credible potential factor, although it is far too early to say it caused this particular derailment. We use steel rails, which expand as temperatures rise, creating significant compressive forces; if the track cannot adequately restrain those forces, particularly where there is an existing weakness or misalignment, buckling can occur.
“Investigators will need to establish the actual sequence of events, examining track geometry, rail stress, ballast, sleepers and fastenings, maintenance records, local temperatures, recent engineering work and any heat-related speed restrictions. We need to understand that the distorted track which was visible after the accident, does not necessarily mean it caused the derailment because the derailing train itself could have caused substantial track displacement.
What is the risk to train travel with extreme temperatures more generally?
“For passengers, the most likely consequence of extreme heat is disruption and individual health risks rather than derailment. Railway operators and infrastructure owners do manage elevated temperatures through additional inspections, rail-temperature monitoring and precautionary speed restrictions.
“However, heat affects more than the rails themselves, including overhead electrical equipment, signalling systems and track foundations. Speed restrictions and cancellations are therefore an important risk-control measure rather than, in themselves, evidence that the system is unsafe.
How well is the UK rail network set up to function at hotter temperatures and during heatwaves?
“The UK railway system has well-established procedures for managing hot weather, but it was fundamentally engineered around the range of climatic conditions historically expected in Britain. There are limits to how far an existing network can accommodate exceptional temperatures without operational restrictions. And notably, these are extreme conditions.
“The bigger strategic question is whether increasingly frequent extreme temperatures mean that historic infrastructure design assumptions need to change. This is becoming an issue of long-term climate resilience, requiring investment in track, signalling, electrification and monitoring as well as operational responses.
Any other assessment based on the information and photographs currently available?
“The visible scale of the derailment and apparent track distortion make the condition of the track an obvious area for investigation, particularly given today’s unusually high temperatures. However, photographs alone cannot establish causation; we need to be very careful at this point not to point fingers and certainly not to the wrong issues.
“The central question is whether any track deformation existed before the train reached that location or resulted from the derailment itself. At this stage, I would therefore describe heat-related track buckling as a credible hypothesis that investigators should examine, not an established explanation for the accident.”
Prof William Powrie, Professor of Geotechnical Engineering, University of Southampton, said:
“It’s important not to speculate as to the cause of this particular derailment so these comments are in the general sense, as I don’t currently know anything about this specific incident. In general terms increased temperatures make continuously welded rails “want” to expand. If they can’t do so, they can build up compressive self stresses that can lead to rail buckling. But this is unlikely unless there is an initial track geometry defect, or at a “weak” or susceptible spot like on a curve or at a switch / crossing, and / or the track is not well supported especially laterally.
“A further “hot weather” problem is that drying of a clay subgrade causes shrinkage, which can make it difficult to maintain perfect track geometry.
“Therefore, this incident could be hot weather related but that the exact causal chain of events needs to be established through proper and thorough investigation, which is under way.”
Declared interests
Peter Dearman: Peter Dearman is an author on the Extreme Heat Taskforce report.
Dr Ioannis Koliousis: No doi
Prof William Powrie: A lot of our research is funded by Network Rail.
For all other experts, no reply to our request for DOIs was received.