Why Do Trains Travel Slower When It's Hot?

Trains are a critical component of modern transportation, connecting people and goods across vast distances. However, the performance of rail infrastructure can be significantly impacted by extreme weather conditions, particularly heat. Why do trains have to travel slower when it is hot? This article delves into the reasons behind this phenomenon, examining the unique properties of rail tracks and the measures taken to ensure safety.

Understanding Rail Expansion

Rail tracks are not static structures but rather have the ability to expand and contract based on temperature changes. This flexibility is crucial for ensuring the safety and efficiency of train travel. In the UK, for instance, rails are designed to be in slight tension whenever the temperature is below 28°C, the typical maximum temperature in summer. This tension prevents the rails from buckling under the scorching heat.

### The Impact of Extreme Heat

When the temperature surpasses 28°C, the rails lose their initial tension and start to expand. This expansion can lead to buckling, a dangerous condition where the rail bends or warps. Buckling is particularly concerning as it can result in track deformations that might cause derailments, which can be catastrophic for both passengers and crew.

Temporary Speed Restrictions

To mitigate the risks associated with hot weather, railway operators often implement temporary speed restrictions on trains. These speed limits are enforced to minimize the stopping distances required and reduce the risk of accidents caused by track deformations.

Possible Risks During Extreme Heat

In extreme conditions, the heat can cause the steel rails to expand to a dangerous degree, leading to deformation or buckling. The expansion forces can overcome the fasteners holding the rails in place, causing tracks to lose their structural integrity. The risk of buckling is higher when the temperature is beyond the range in which the tracks were initially stressed during installation. Such deformations can also lead to track kinking and derailments, making speed restrictions a necessary safety measure.

Managing Track Deformation Through Speed Restrictions

### Imagining Extreme Conditions

Imagine the following scenario: The tracks are already burning hot and undergoing thermal expansion. A train putting enormous pressure on the tracks can exacerbate the deformation further. The faster a train travels, the more friction it generates, contributing to even more track heating. Additionally, the heat generated by a fast train is trapped due to the low surface area in contact with the wind, resulting in higher surface temperatures than internal track temperature. This scenario can lead to a point where the track can either deform or endure a thermal shock, potentially causing significant safety issues.

### An Extreme Example

This is an extreme example of the impact of heat on railway infrastructure. Traditionally, trains made a characteristic clicking sound as their wheels passed over gaps between rail lengths, but advancements like welded track have reduced these gaps and associated vibrations. However, on high-speed lines, where traffic is heavier and speeds are faster, the risk of encountering excessively hot conditions is higher. When the heat levels reach critical stages, reduced speeds are enforced to minimize the risk of accidents. Increased inspections and, in some cases, line closures, may also be necessary to ensure the safety of the rail network.

Conclusion

Extreme heat poses a significant threat to the reliability and safety of railway systems. By understanding the principles of rail expansion and the measures taken to ensure track stability, we can appreciate the importance of speed restrictions during hot weather. This knowledge not only enhances safety but also contributes to the uninterrupted and efficient operation of modern railway systems.