It is virtually impossible to take the train in the UK right now. Nearly all of the services on the National Rail network have reduced service or “do not travel” warnings. Transport for London, which runs the London underground and overground services, has also advised people not to travel. The service disruptions are, of course, due to the unprecedented heat wave across the UK. Temperatures have topped 40 Celsius or 104 Fahrenheit, setting all kinds of records across the country.
High heat doesn’t always disrupt train travel. Trains run through some of the hottest parts of the world like the Mojave and Sahara Deserts. They can do this because it is not so much the temperature itself that causes problems but the difference between the actual temperature and the temperature the system was designed to handle.
The problem for the UK and anywhere else suddenly experiencing triple-digit temperatures (on the Fahrenheit scale) for the first time is that rail networks—and roads, for that matter—can only be engineered to accommodate a certain range of temperatures. Mild climates have erred on the side of accommodating cold spells rather than heat waves. Now that they have to deal with both, they have a genuine problem that won’t be easy, quick, or cheap to solve.
Among the biggest concerns during extreme heat is buckling, when the track is literally bent out of shape. Most train tracks these days are made up of long strips of steel called continuous welded rail (CWR). These strips, which are welded together when installed, can often be miles long. CWR is popular because it is cheaper to maintain and provides a smoother, quieter ride than the traditional steel strips that gave trains the clickety-clack noise. But an unfortunate unintended consequence is the long steel rails can make buckling more extreme.
Steel expands and contracts as the temperature changes. The longer the steel rail, the greater the force of that expansion and contraction. The ties—or “sleepers” as they’re called in the UK—are the (typically wooden) boards laid across and underneath the tracks. Normally, these ties keep the tracks in place during normal temperature fluctuations. And ballast, usually crushed stones or gravel, gets packed underneath and between the sleepers to help keep everything stable.
Railroad engineers will rate the system up to a certain temperature, known as the stress free temperature, which is typically based on the region’s historic climate. This is the “neutral” temperature at which the rail exerts zero sideways force or pressure due to expansion or contraction. This is something the railroad and the manufacturer decide on before they put the rail in place.
In extreme heat, rail beds sitting in the sun all day will bake about 20 degrees Celsius hotter than the air temperature. The steel rails expand and exert force on the ties and ballast. Trains running over the tracks will add to that force and the faster the trains go, the more force on the tracks, which is why most rail services in the UK are under slow speed warnings if they’re running at all. If the force becomes too much for the ties to handle, the tracks will buckle, will become permanently damaged, and will need to be replaced.
Unfortunately, due to climate change, which makes extreme temperatures both hot and cold more likely, the UK’s rail network is in a bind. Rails can typically only handle a certain range of temperatures on either extreme from the neutral temperature the rails were designed to accommodate. If the UK undertakes the decades-long process of rating its network for hotter stress free temperatures, it will make buckling in the winter more likely, according to Network Rail which owns and maintains the country’s rail network. It will also cost hundreds of millions if not billions of pounds, as studies have consistently found.
There are other options like using a concrete ballast system, but concrete also expands and contracts in extreme temperatures, and roads made of concrete buckle, too, for the same reason steel rails do. Engineers can design spaces for the concrete to expand into like they do with steel, but concrete can be four times more expensive than traditional ballast systems, according to Network Rail. More frequent track inspections and temperature checks can help detect buckling before it takes place, but that is a necessary safety measure, not a means of maintaining full service during heat waves.
In the meantime, the UK and other countries experiencing frequent record temperatures are going to be stuck with infrastructure that simply can’t handle it. As Grant Shapps, the UK’s transport secretary, told the media when asked if the transport system could cope with extreme weather, he replied, “The simple answer at the moment is no.”
Correction: A previous version of this article misstated how much hotter rails get than the ambient temperature when in direct sunlight. It is 20 degrees Celsius, not 10, and definitely not 50 degrees Fahrenheit.