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Scientists comment on a train derailment in Essex and a possible link to extreme heat.
Dr Frederic Cegla, Reader in Mechanical Engineering, Imperial College London, said:
“Heat has long been recognised as a potential cause of rail derailments: high temperatures can cause rails to expand and, in certain circumstances, buckle, affecting the safe operation of the railway if it cannot be mitigated. Research has shown a strong relationship between daily air temperature and rail buckling incidents, with the vast majority occurring during the summer months. While each rail network will have its own peculiarities they will all generally become more susceptible to buckling with a general increase in ambient temperatures, and the UK rail network will be no exception.
“A 2025 study showed a clear correlation of an increase in rail buckling events in line with hotter temperatures. It would therefore be no surprise to also see an increase in buckling events in the south of the UK during periods of extreme heat as has been experienced over the last two days. However, one needs to await the outcomes of the official investigations into the causes of the derailments before jumping to conclusions.
“Since temperature effects on rail are a well-known phenomenon, I am very certain that the people overseeing the UK rail system are aware of the issue. They will have mitigation strategies that take the many important factors into account (temperature, track condition, traffic levels, line geometry, train operations, maintenance, etc.). No doubt the high temperatures that we have been experiencing are at the extreme end of what has been experienced in the past and one needs to be prepared for the case in which they will become more frequent and even the status quo in the future.”
John Lawrence, Chair of the IET Railway Technical Network, said:
“At this stage it’s too early to say what has caused the Essex derailment. RAIB are already on their way and will work to establish the cause and identify any lessons from the incident.
“There are a number of potential causes that investigators will consider. It could be heat-related, an underlying structural issue with the track sub-base, a train wheel problem, a track defect or maintenance issue. As the derailment is reported to be near a station, a defect in the pointwork could also be a factor.
“If the issue is loss of track alignment, then it could be heat expansion in the rails causing them 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 potentially cause a buckle beneath the train.
“Another possible cause is soil moisture deficit, particularly in areas built on clay. Prolonged dry periods can cause shrinkage of the track bed and distort the rails, which may then move under train loading.
“However, it would be wrong to conclude that hot weather is responsible before the investigation has been carried out. The railway has a range of mitigation measures in place, including remote track temperature monitoring, speed restrictions during extreme temperatures, routine inspection and maintenance, and ongoing work to understand and manage these risks.
“The immediate priority will be safety. Rail traffic in the area will be stopped, emergency services and railway response teams mobilised, and the site secured both to protect people and preserve evidence. RAIB will then look at all matters surrounding the incident, including infrastructure, train maintenance, operational factors and any wider contributing issues before reaching conclusions.
“From the information available so far, recovery of this train may be more straightforward if only a single bogie has come off the track. The railway has specialist equipment available to re-rail vehicles in these circumstances. Incidents involving a single bogie leaving the track occur more frequently in the freight sector than in passenger operations because of the different dynamic loads, wheelset arrangements and failure modes that can occur, such as bearing failures.
“While two derailments in a short period will naturally attract attention, it would be premature to assume they are linked. The investigation will determine whether there are any common factors or whether they are separate incidents with different causes.”
Dr Ioannis Koliousis, Associate Professor of Logistics & Supply Chain Management, Cranfield School of Management, said:
“The Essex incident appears, from the information and photographs currently available, to be considerably less severe than the Lewes derailment. The rear carriages derailed but remained upright and there are currently no reported injuries. Images show the train displaced from the running rails and there also appears to have been an interaction with the overhead-line infrastructure. However, none of this tells us what initiated the derailment; damage visible after an accident can be either a cause or a consequence.
“It is premature to say that hot weather caused either derailment. RAIB investigators will reconstruct the sequence using physical examination of the track and train, on-train data, signalling information, CCTV, maintenance and inspection records, witness evidence and weather information. Given the temperatures, heat and possible track movement are clearly hypotheses that should be tested, but investigators must also consider vehicle, wheelset, points, track and other infrastructure failures. The important distinction is between saying that heat is a credible risk factor and saying that heat has been established as the cause.
How common are derailments? Could two in such a short period simply be coincidence? Are they more common in hot weather?
“Serious passenger-train derailments are very rare on the British railway. To put this into perspective, the ORR recorded five potentially high-risk derailments on the mainline in 2024–25, only two of which involved passenger trains, in a year with more than 1.7 billion passenger journeys. So, two passenger-train derailments within less than 24 hours is certainly unusual that warrants further investigation.
“It could still be coincidence, and we should not infer a common cause simply because the incidents occurred close together. However, when two unusual events occur during the same period of exceptional heat, it is entirely reasonable for investigators to examine whether there is a common environmental factor. We know that high rail temperatures increase the risk of track buckling, and RAIB has previously identified thermal expansion as the cause of a derailment. That demonstrates the mechanism is real, but it does not establish that it happened in either of these cases.
How well is the UK rail network set up to operate at these temperatures and during heatwaves?
“The UK railway is well set up to manage heat as a safety risk, but that is different from being able to maintain normal performance at every extreme temperature. Network Rail uses weather forecasting, thousands of trackside temperature probes, additional inspections and precautionary speed restrictions. Rails in direct sunlight can become around 20°C hotter than the air, and Network Rail says most of the network can operate at rail temperatures up to around 46°C.
“The two incidents inevitably raise a wider resilience question. Britain’s railway was designed and maintained around a historic range of climatic conditions; as extreme temperatures become more frequent, the question is whether the engineering assumptions, inspection regimes and intervention thresholds remain appropriate. The challenge is therefore not simply responding to individual heatwaves but progressively adapting our infrastructure to a changing operating environment. ORR is already requiring Network Rail to strengthen its weather-resilience and climate-adaptation planning.”
Comments on the Lewes train derailment below:
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.
“Network 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.
“Network 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
Dr Frederic Cegla: Director of Sonobotics Ltd, Embed Bio Ltd, CodeAcq Ltd.
Scientific Advisory Board member of Verasonics Inc.
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.