Why are trains cancelled when the weather gets too hot?
- Credit: Sonya Duncan
With almost dozens of trains cancelled between Norwich and London yesterday because of the hot weather, Greater Anglia has provided details which help explain the disruption.
Why don't rails get hot in Europe?
The truth is, they do. However, in countries which have been traditionally hotter than ours, rails are stressed to be able to withstand higher temperatures.
Rails in the UK are stressed to 27 degrees, the UK mean summer rail temperature. The rail can withstand some degree of change in temperature either side of this. However, when the air temperature is 30 degrees, the temperature on the rail can actually be up to 20 degrees higher, exceeding the maximum temperature our rails are designed to cope with.
What happens when rails get hot?
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The problem is that when steel rails get hot, they expand and this can cause a type of signalling problem called a track circuit failure and in extreme cases can cause a buckled rail.
If a rail buckles then the train service will not be able to run at all until the buckled rail is replaced with a new one.
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When Network Rail assesses that the temperature of the rails is going to exceed what it is designed to cope with they put speed restrictions in place to help prevent buckled rails.
A train going over a hot rail at a slower speed exerts less pressure on the rail and will help to prevent it buckling. Because overhead power lines also expand in the heat and sag, it also prevents the train's pantograph from pulling the sagging wires down.
It means have trains have to run more slowly, and there will be fewer trains as a result, but this is better than causing a bigger problem and then being unable to run any trains at all.
Why don't we replace all our rails with ones stressed for higher temperatures?
The UK, and particularly East Anglia, is seeing hotter summers these days so this may seem like the obvious solution. The problem is that if UK rails were stressed to the same degree as those in very hot countries, there would be the risk of rails fracturing when the rails contract as the temperature drops in winter.