What happens to railways in the heat?

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.

How do hotter countries cope?

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.”

One option, in theory, could be to re-stress rails between seasons, loosening and retensioning them so the stress-free temperature is set higher for summer and lower for winter, rather than fixed year-round.

Network Rail says: “In Britain, the variations in short-term weather and long-term climate mean that it is neither practical nor cost-effective to implement these measures permanently.”

Another option is to lay track on solid concrete slabs, as in some countries, rather than the more common bed of sleepers and ballast. However, slab track costs about four times as much to install as standard ballasted track, which is why it remains rare across most of the UK network.

What can be done?

One idea under discussion is raising the temperature British rails are built to withstand. The post-2022 taskforce said that on modern, well-supported track the stress-free temperature could rise from 27C to 30C.

“You might say we should actually have a different stress-free temperature in the south of England compared to, say, the north of Scotland,” Powrie said. However, higher stress-free temperature also leaves rails more vulnerable in a cold snap.

Simpler measures are already in use. Some sections of track are painted white to reduce how much heat they absorb. Network Rail says a rail painted white is typically 5-10C cooler.

Both tactics may be necessary as summers get hotter – something that will be increasingly normal amid the climate crisis. All five of the UK’s warmest summers on record have happened since 2000, according to the Met Office.

Monitoring for faults remains difficult, though. The Network Rail taskforce found that fewer than half of the track buckles recorded in 2022 had been identified before they happened.

Part of the issue is the sheer amount of data modern trains generate. Many now carry sensors that pick up rough riding or unusual vibrations, but sifting through it all is a challenge that needs to be automated, experts say.

Much, therefore, still relies on judgment rather than data – for example, a train driver detecting rough riding on one part of the line and reporting it.

As a temporary measure, speed restrictions were imposed on trains in southern Essex in the aftermath of the Wickford derailment.

Did the Lewes derailment happen because of the heat?

It’s impossible to say for sure. An interim report by the Rail Accident Investigation Branch (RAIB) found that a “track geometry irregularity” existed before the derailment, and said investigators had since received reports of rough riding from passengers on that stretch of line.

Sasidharan said early evidence pointed to the track itself. “The initial indication of the derailment … is that it’s a track infrastructure fault, most likely,” he said.

At the site of the derailment, the embankment is built on clay, which shrinks more than other types of soils as it dries out. The RAIB said the clay was “sensitive to variations in water content”, and there was evidence it had shifted before the derailment, though not enough to amount to a significant slip.

Powrie said the location had several things working against it at once, making it a case of what safety experts call the “Swiss cheese” theory of accidents, where one only happens when several small failures line up together.

The derailment happened on a bend – where more force is exerted on the track – and close to where the track leaves a bridge and moves on to the embankment.

Powrie said these transition points were a natural weak spot because the track had to cope with a sudden change from something rigid to ground that could shift, especially when it was dried out. “It wouldn’t surprise me if the defect turned out to be related to the dryness,” he said, “but I don’t really want to speculate.”

Network Rail has said it is “fully committed to supporting the continuing investigation and acting on any lessons that emerge from its findings”.

It said: “Safety is and will always remain our highest priority. While Britain’s railway is one of the safest in the world, when incidents like this occur it is essential that we understand exactly what happened and identify any lessons that can be learned.

“Given the complexity of the issues being examined, it is important that the independent investigation is allowed to reach its conclusions. We will continue to cooperate fully with the RAIB throughout this process.”