Problems including derailments have hit network during one of the hottest and driest summers on record
Britain’s railways have faced a string of heat-related problems this summer, including a derailment near Lewes in East Sussex two weeks ago that injured 30 people, and another the next day when the rear carriages of a Greater Anglia train derailed “in an upright position” at Wickford railway station in Essex, with no reported injuries.
The problems came during one of the hottest and driest summers on record, with a series of heatwaves, and drought declared across large parts of England and Wales. Britain’s railways, built for a cooler, wetter climate, have been put under pressure. Network Rail said the summer had “presented exceptional challenges to the railway”.
Here’s what we know about how heat affects the rail network and what can be done to tackle it.
Steel expands when it gets hot, and the long, welded steel sections that make up modern railway tracks are no exception. “The classic concern for railways with hot weather is that the track might buckle,” said William Powrie, a professor of geotechnical engineering at the University of Southampton.
As temperatures rise, the rails expand, but their continuous design means there is little room for movement. “If they haven’t got anywhere to go, they end up generating compressive stresses,” Powrie said. “The problem with any sort of structure which has got a compression in it is that it can fail by buckling.”
The consequences can be serious. Just two days of record heat in 2022 led to almost 7,700 cancelled trains and an estimated £30m in costs, according to a taskforce set up by Network Rail after that heatwave.
On a hot day, the rail can reach much higher temperatures than the air. Network Rail says most of the network can operate when track temperatures reach up to 46C, roughly equivalent to an air temperature of 30C. But rails have been recorded at temperatures as high as 51C.
The sleepers and ballast that rails sit on can help stop them from buckling, keeping the track in place – but long periods of heat and drought can dry the soil beneath them. That soil can shrink when it dries, a problem given it “is essentially the foundation for anything on top of it”, said Manu Sasidharan, an assistant professor of infrastructure asset management at University College London.
The weight of passing trains makes the problem worse, putting pressure on the weakened soil and causing it to settle unevenly. Whatever sits on top, including the track, “will follow suit”, Sasidharan said.
While tracks and stations have been continually updated over the decades, some embankments, tunnels and other structures date to the 1800s, when much of the network was first laid.
To stop rails from buckling, engineers lay them slightly stretched, a process called pre-stressing. It means the rail has some give in it before it starts expanding in the heat.
In the UK, rails are stretched so that they do not undergo compression until the air temperature passes 27C. That figure sits between the UK’s highest and lowest seasonal rail temperatures, protecting against buckling in the summer and breaking under tension in the winter.
Other countries set that figure differently. In parts of Australia and the US, the so-called stress-free air temperature can exceed 35C. In southern Europe too, where winters rarely get as cold as in the UK, rails are designed for more heat, since there’s less risk of the rail snapping in the cold.
“If you’re looking at Italy, where you don’t really get very low winter temperatures, then you can afford to increase the stress-free temperature,” Powrie said. “The problem is the range of temperatures that we have [in the UK] … in the winter, they go into tension and that can cause the rails to break.”
