Why are Britain’s roads melting and the rails tensing in the heat?  |  Infrastructure

Why are Britain’s roads melting and the rails tensing in the heat? | Infrastructure

Extreme temperatures have led to widespread problems and disruption to UK railways, with low-speed trains and main lines closed. Runways at airports and some roads have also shown that they can be exposed to heat.


Steel rails expand and tend to get stuck in the heat – regardless of climate. According to Network Rail, railways around the world are designed to operate within a range of 45C, according to local conditions. In the UK, steel rails are “biased” to summer temperatures of 27C, while in countries with warmer climates, rails are biased to higher temperatures.

Sleepers and ballast will keep the rails in place during the British winter and summer. When the temperature reaches 40C, the rails can reach 60C and expand and tense. A train running fast over rails can speed up this process through the heat caused by friction, and can be in greater danger if it should break up – hence the widespread speed limits.

The overhead lines on electrified routes also expand and sink in the heat, and contract in cold weather. Engineers have solutions, with the voltage automatically attenuated by a pulley system. But eventually the counterweights hit the ground and the wires hung – making them more likely to be entangled in a pantograph, the device at the top of the train that draws power from the lines.


Highways and strategic roads are built with modified asphalt surfaces that – so far – will not start to melt, be resistant above 60C or a corresponding air temperature of 40C, according to National Highways. However, basic asphalt materials used on local roads – the vast majority – can begin to soften at temperatures of 50C. At the time, Professor Xiangming Zhou, head of civil and environmental engineering at Brunel University, said: “The road can become soft and greasy, and it is difficult for cars to brake.” This is the reason why the municipal councils have put stray cars, more often used in icy weather, in readiness to cover roads with sand and dust. Asphalt and asphalt are cheaper and less abrasive on tires than some materials, he says, but since they are black, they tend to heat up faster in the scorching sun.

About 4% of UK roads are built of concrete, which is more popular abroad for motorways and motorways and may be more resistant, but not immune to extreme temperature problems, as the closure of the A14 shows. The two carriageways near Cambridge were built with asphalt over old concrete slabs that expanded and bent in the heat, creating a bump that was sufficient to close the road overnight for emergency repairs.

Rick Green, of the Asphalt Industry Alliance, says that a way to cope with all temperatures is “a significant challenge for design engineers”. In extremely high temperatures “the surface does not melt, but the bitumen in it can soften”, “increases the risk of deformation”.

Airport runways

Again, some may be specific – but Luton’s asphalt was the problem when temperatures rose to the mid-30s, Zhou said. In the words of the airport, “high surface temperatures caused a small section to rise” – a buckle in the runway that engineers fixed within hours, but which still caused major disruption to passengers. While local roads are often in the shade of trees and houses, the runways are completely exposed and under additional heat stress from planes landing and taking off. Repairs and maintenance are frequent.

Heathrow, which was even hotter than Luton on Monday, also had a runway problem last week, as overnight repair work was not completed in time for the planes to land. However, it has two runways and was not forced to stop operations.

So what is the solution?

Network Rail already spends hundreds of millions of pounds annually on reducing climate change. Most, however, are to counteract erosion or damage through precipitation or storms. Future infrastructure can be measured against a warmer climate – but then it may be more prone to faults and cracks in cold winter weather when the rails contract. Some track materials, such as concrete sleepers, are more resilient at wider temperature and conditions – and significantly more expensive.

Rails are already painted white in critical places to combat heat. Countries with extreme weather conditions make much broader seasonal adjustments to track, which is time consuming and costly. Air conditioning was not a standard feature for older trains still running. Resilience will be an economic and political choice – and it may be that a few days of heat failure each year are seen as preferable to the bill for modifications.

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