The UK rail service has been hit by numerous delays and high levels of train cancellations during the heat wave. The entire east coast line between Edinburgh and London was closed for hours on July 20 due to the heat. Why can’t Britain’s railways withstand the heat?
Why is heatwave such a burden on UK railways? Like most building materials, steel, from which rails are made, expands when the air temperature rises. When this movement is restrained by the anchor, which holds the rail and sleeper (the rectangular supports under the rails) still, internal stresses build up causing the compression to bend the rail.
Trains cannot run on railways with bends. In the US, sun bending has caused more than 2,100 derailments in the past 40 years, equivalent to about 50 derailments per year.
How are UK railways heat protected?
When the air temperature reaches 30°C, UK railways sitting in the sun can reach 50°C. In the UK, a temporary speed restriction is imposed when the tracks reach this temperature, as trains put less pressure on the tracks when running slower.
Some railroad maintenance crews paint the tracks white in the summer to cool them down. According to Network Rail, this can reduce the track temperature by 5°C, up to 10°C. Network Rail workers started painting the tracks days before July’s heat wave.
Modern rail systems are getting better at keeping the rails fixed. Most rails are made up of long pieces of metal that are stretched and welded together, which helps keep them in place. This reduces the chance of buckling. When a track is made up of short rails clamped together, Network Rail leaves small spaces between each to allow them to expand in hot weather.
Electric trains are often powered by overhead wires which can also be damaged by hot weather. Heat can cause strands to expand and sag. Too much slack will damage the feeding equipment. A pulley system keeps overhead wires taut and can compensate for sag, he writes Tech Xplore.
Why can’t Britain’s railways withstand the heat?
As temperatures rise to unprecedented levels, even Spanish rail lines, which usually do well in the heat, are giving way and services are being cancelled.
Continuous welding rails, which are in standard use around the world, including the UK, are optimized to operate around a certain ‘stress free temperature’ (SFT). The higher this temperature, the hotter the rails can get without buckling.
In the UK, the SFT is 27°C, the usual average summer rail temperature. In the US, standard values range from 35°C to 43°C.
The Rails Dilemma
If the UK’s SFT was higher then the railways could operate in warmer weather as the rails would not expand as much in warmer temperatures. But if the rails are made for too high an SFT, then they won’t be able to handle the colder temperatures they experience in winter. This would create tremendous stresses and the rails could destroy the anchorage.
Railways in some countries are able to cope with such high temperature variations by using solid concrete slabs to limit the higher forces created. But track plates cost 4 times more to install than standard ballasted tracks.
With climate change, rails in many countries will more often reach temperatures they were not designed to withstand, threatening public transport on an unprecedented scale. Britain just had its hottest temperature ever recorded at 40.3 degrees Celsius, well above the previous high of 38.7 degrees in 2019.
Railways equipped with weather sensors
During the construction of HS2, the new high-speed railway being built between London, Manchester and Leeds, bridges and overhead wires were fitted with sensors to collect weather data, including air temperature. This data will be used to create digital twins of HS2, enabling a new type of maintenance that can predict high temperatures and other types of disruptive weather.
A digital twin is a digital representation of the rail network. This would allow engineers to simulate future scenarios for the UK rail network and predict what might happen if high temperatures are forecast and take preventative measures. In this way, the huge number of cancellations that happened during the heat wave could be reduced.