Why Lifted Railway Tracks Curve in the Summer Heat

On some hot afternoons, railroad crews will slow or even stop trains for a reason few passengers notice: the metal tracks themselves are shifting. If you stand beside a railway in July, you might notice spots where the rails no longer form a perfect line. Instead, they bulge to the side, creating what engineers call a "sun kink."

Rail tracks are made of steel, and steel expands as it heats. A 100-foot section of track will grow by almost half an inch when the temperature rises by 30°C. That may not sound like much, but with rails clamped tightly to the ground, the built-up force, several tons per rail, has nowhere to go. The metal pushes sideways into the weakest spot it can find. The design of the track itself is a delicate balance: rails must be anchored firmly so trains don’t shift them, yet not so rigidly that a hot day causes dangerous buckling.

To handle these risks, most railways now use continuous welded rail, those sleek, seamless ribbons you see today. With fewer expansion joints, engineers must pre-stress the rails. When they’re installed, the rails are stretched and fastened at what’s called the "neutral" temperature, typically matching the area’s highest annual average. This helps the rails tolerate the summer heat without creating even more risky expansion.

Even with all these precautions, extreme temperatures sometimes force train crews to slow down and send out teams to watch for any signs of track movement. On very hot days, crews may even hose down rails to cool the steel. Every time the tracks shift or kink, it becomes a contest between the physical world and careful monitoring. A single unnoticed sun kink can be enough to derail a train.

The next time you notice those mirrored rails on a hot afternoon, think about the quiet, steady force always at work. Holding that line is what keeps trains running straight and safe, day after day.