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July 29, 2019

Why UK railways can’t handle a heatwave - and how technology could help

World

July 29, 2019

Commuters are facing rail chaos today as a ferocious heatwave that has broken temperature records across Europe hits its peak in the UK.

Temperatures are expected to hit a record 39C in London and, as with most extreme weather, trains could come to a standstill.

The main problem facing Network Rail is bending tracks. Around 20,000 miles (32,187km) of steel track criss-crosses the country, much of which is exposed to sunlight.

“In rails on a hot day, they are exposed to very high temperatures in the sun (if you feel your car roof on a sunny day it will be much hotter than the air temperature),” explained Dr Darren Hughes, Associate Professor in Materials and Manufacturing, University of Warwick.

“The rail expands, pushing the rails sideways along the length and causing risk of derailment.”

Just a few hours of hot weather is enough to cause rails to bend. But why is this still a problem in the UK? And can technology provide a solution?

Tracks in direct sunshine can be as much as 20C hotter than the air temperature. As temperatures soar steel rail absorbs heat, causing it to bend or buckle.

“In the old short lengths of rail this effect was accommodated by having regular expansion/contraction joints,” said Professor Peter Dobson, Principal Fellow at the University of Warwick.

“In the modern continuous welded rail track, the rail is stretched when it is laid and welded so that it is under some tension in ‘ordinary’ average temperatures and in reduced tension in ‘ordinary’ hot weather – but the rail does not change its length.

“However, if there are prolonged periods of extreme heat, the expansion may overcome the original induced tension and the rails will bend.

“The track layers have to judge the tension required according to the maximum and minimum temperatures that the rail will experience. This is a fine balance, because overstretching may cause rails to crack in extreme cold weather.”

Overhead lines can also expand and sag in extreme heat, bringing a risk of passing trains pulling them down.

Tracks have a “stress-free” temperature of 27C (80.6F), which Network Rail said is the UK mean summer rail temperature.

It said more than three-quarters of its track is on concrete sleepers, which, when “fully compliant” with its standards, can withstand rail temperatures of more than 59C (138.2C).

But some sections of track are not designed to cope with that level of heat and are at risk of buckling.

The company said it can roll out extreme weather action teams (EWATs) to ensure passenger safety and keep trains running during hot weather.

Weather conditions are monitored through specialist forecasters and the use of “mini weather stations” and track-side probes.

As train movements also exert force on tracks, slower speed restrictions can be introduced to reduce the chances of buckling, with the side-effect of disrupting timetables.

Network Rail said it paints some track white as this makes it less heat-absorbent and reduces the temperature by 5C or 10C.

Where a stretch of track is composed of short rail sections bolted together, gaps are left to allow for expansion.

Network Rail said other nations choose a higher stress temperature limit for their tracks, depending on the climate.

Countries with extreme weather conditions adjust their tracks between summer and winter, the company said.

This can include inserting concrete slabs which can better contain track forces than sleepers and ballast.According to the company “variations in short-term weather and long-term climate” in Britain mean “it is neither practical nor cost-effective” to implement such measures permanently.

It said solid concrete slab tracks costs about four times as much to install as standard ballasted track.

Researchers examining the issue of railway tracks buckling have suggested a variety of emerging technologies which could be used to reduce the amount of disruption in hot weather.

One suggestion is to make use of thousands of “Internet of Things” sensors connected to the track at regular intervals in order to more closely and accurately measure the heat of the rails.

Sensors such as this have been expensive in the past, but developments in the chips used has seen prices become more affordable in recent years.

The roll-out of 5G networks across the UK will also make it easier for companies to deploy more sensors, as the technology allows up to 1m devices to be connected per square kilometre, a rise from the current cap of 60,000 devices for the same area on existing 4G networks.

A 2016 study into the buckling of rails in South East England published by researchers at the University of Birmingham suggested the use of these sensors.

They could “reduce the time needed to respond to a heat-related fault and ultimately the length of disruption to service,” the researchers said.

Reducing the weight on the tracks as trains pass over them could also help to prevent buckling.

A research project run by the University of Huddersfield is investigating the use of 3D printing to replace metals currently used to manufacture trains.

Replacing metal train bodies and parts with 3D-printed carbon fibre could substantially reduce the weight on tracks.

The research project is investigating whether carbon fibre could be used to replace metal, and is also examining selective laser melting, a form of 3D printing which fuses metallic powders together.

A Chinese business, CRRC Changchun Railway Vehicles, has developed a carbon fibre train body, which it says weighs around 35pc less as a standard steel body.

“The problem is the tendency of rails to expand in heat and contract in the cold,” said William Powrie, Professor of Geotechnical Engineering, University of Southampton.

“They’re generally laid in a stress free temperature but as the rail temperature rises above the stress free temperature the rails go into compression and this makes them more prone to buckling.

“Raising the stress free temperature (by stretching the rails more when they’re laid) would put more tension into the rails when the temperature drops, making them more likely to break in cold weather. But breaks often initiate from flaws in the rails and we are now much better at eliminating these during manufacture and detecting them in service than we were 20 years ago.

“We can improve the lateral resistance of the track by enhancing the performance of the ballast eg. reducing the shoulder slope or changing the grading, or by sleeper modifications eg. wings or plates, but these are used mainly only on tight curves in the UK at present.

“A further potential application of technology would be, for example, to use optical fibres to monitor the actual longitudinal stress/stress free temperature (which, although in theory uniform, in practice varies hugely).

“That might enable us to manage speed restrictions (the main aim of which is to reduce variable and lateral loads) more selectively.”

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