With the current heatwave and an amber alert across much of the country, rail passengers have felt it: delayed departures, stretches run at reduced speed, lateness that builds up station by station. It is not improvised chaos, nor an operator being overcautious for the sake of it. When temperatures climb, the rails themselves can pass 50C, and trains are made to slow down for one very precise safety reason. Behind that delay announcement lies a piece of physics that applies to every railway in Europe.

Why rails buckle in the heat

Rails are steel, and steel expands as it heats. A 32C day in the shade easily means rails at 50 to 55C in direct sun, because the dark metal soaks up and holds the radiation for hours. Modern track is welded into long continuous lengths, without the old joints that once left room for expansion, and it is pre-stressed when laid to a reference figure known as the stress-free temperature. Beyond that point the expansion has nowhere to go, and the rail risks buckling, meaning it distorts sideways. That is the failure engineers work hardest to prevent, and in extreme cases it is visible to the naked eye as a wave in the steel.

The problem bites hardest in countries with historically mild summers, where lines were built for a cooler climate than today's heatwaves deliver. It is no coincidence that the same speed restrictions appear from Britain to northern Italy whenever the mercury breaks its usual bounds.

Why slowing down keeps it safe

Running trains more slowly does not cool the rails, but it cuts the forces a train loads onto track that is already under strain. A lower speed reduces the sideways push on the rails and gives engineers the margin to act if a section shows the first signs of distortion. Through the hottest hours, Network Rail steps up inspections, watches the critical spots and imposes targeted speed restrictions on the most sun-exposed stretches. It is a call made out of caution: a few minutes of delay is better than the risk of a derailment.

There is an angle that speaks directly to anyone who follows energy. The very heat that bends the rails also piles pressure on the electricity grid. Through the afternoon, air conditioning and fans run flat out, demand hits its peak and wholesale prices rise: it is worth seeing how electricity prices move on days like these. Trains draw more too, because carriage air conditioning works at full tilt just as they are forced to run slower. Extreme heat does not just slow the trains, it tests every piece of infrastructure we take for granted.

For passengers the practical takeaway is simple: on the hottest days, build in some delay, check the live times before you leave home and carry water with you. And if the fans and cooling at home are running non-stop to cope, it is worth knowing where the savings are, starting with our guide on how to save electricity. The rail that expands and the bill that swells share one cause: heat pushing every system past what it was designed for.