Heatwave causes railway track buckling and delays

Britain's railway network has experienced numerous heat-related issues this summer, including derailments and significant service disruptions. These problems have emerged during one of the hottest and driest periods on record, placing considerable strain on infrastructure built for a cooler climate. Network Rail acknowledged that the summer "presented exceptional challenges to the railway."

How heat affects railway tracks

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When steel railway tracks heat up, they expand. The continuous welded sections of modern tracks have limited space for this expansion, leading to the development of compressive stresses. Professor William Powrie of the University of Southampton explained that these stresses can cause the track to buckle. The consequences can be severe; in 2022, just two days of record heat resulted in nearly 7,700 train cancellations and an estimated £30 million in costs, according to a Network Rail taskforce.

Network Rail indicates that most of the network can function safely with track temperatures up to 46C, which is roughly equivalent to an air temperature of 30C. However, track temperatures have been recorded as high as 51C. The sleepers and ballast supporting the rails can help prevent buckling, but prolonged heat and drought can dry out the underlying soil. Manu Sasidharan, an assistant professor at University College London, noted that this dried soil can shrink, compromising the foundation for the track. The weight of passing trains exacerbates this issue, causing uneven settling.

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To mitigate buckling, engineers sometimes lay tracks slightly stretched, a process known as pre-stressing. In the UK, this is set so that rails do not experience compression until the air temperature exceeds 27C. This figure is designed to balance protection against summer buckling and winter tension. In contrast, countries like Australia and the US set their stress-free air temperature above 35C, and southern European nations have similar adjustments due to milder winters. Professor Powrie highlighted that the UK's wide temperature range presents a challenge, as rails can become vulnerable to breaking under tension during cold periods.

Network Rail has stated that permanently adjusting these measures to account for short-term weather and long-term climate variations is neither practical nor cost-effective for Britain. Laying track on concrete slabs, common in some countries, is an alternative but costs approximately four times more than standard ballasted track, making it rare across the UK network.

Potential solutions and ongoing measures

One proposal involves raising the temperature threshold that British rails are designed to withstand. The post-2022 taskforce suggested that modern, well-supported tracks could potentially have their stress-free temperature increased from 27C to 30C. However, a higher stress-free temperature would also increase vulnerability during cold weather. A simpler, existing measure involves painting sections of track white, which Network Rail reports can reduce rail temperatures by 5-10C by decreasing heat absorption.

These tactics may become increasingly necessary as summers continue to get hotter, a trend linked to the climate crisis. The Met Office has reported that all five of the UK's warmest summers on record have occurred since 2000. Monitoring for track faults remains a challenge, with the Network Rail taskforce finding that fewer than half of track buckles in 2022 were identified beforehand. The sheer volume of data generated by modern trains, while useful, requires automated systems for efficient analysis. In the immediate aftermath of the Wickford derailment, speed restrictions were temporarily imposed on trains in southern Essex.

Investigation into specific derailments

The exact cause of the derailment near Lewes remains under investigation. An interim report from the Rail Accident Investigation Branch (RAIB) identified a "track geometry irregularity" present before the incident and noted passenger reports of rough riding on that section. Mr. Sasidharan suggested that initial evidence points towards a track infrastructure fault. The embankment at the derailment site is constructed on clay, which is prone to shrinking when dry and is sensitive to water content variations. The RAIB reported evidence of embankment movement prior to the derailment, though not to the extent of a significant slip.