Why does hot weather cause rail delays?

A train moves through heat haze in Ashford, Kent. Credit: PA

As the hottest weather so far this year hits parts of the UK, train passengers are being warned of further train delays caused by buckling rails.

Network Rail's extreme weather action teams are being activated across the country to monitor "vulnerable locations".

On Monday and Tuesday evening, travellers attempting to leave London Waterloo were met with long queues in the station and reported "hot trains not going anywhere".

But why does this happen?

When Britain enjoys a summer heatwave, steel rails exposed to direct sunshine can become 20°C hotter than the air temperature, according to Network Rail.

Because rails are made from steel, they expand as they get hotter, and can start to curve – the technical term is called 'buckling'.

When this happens, lines become impassable and close for repairs, which can't usually happen until temperatures drop again.

Most of the network can operate when track temperatures heat up to 46°C – roughly equivalent to air temperature of around 30°C – but rails have been recorded at temperatures as high as 51°C.

Managing director of England and Wales at Network Rail, Andy Thomas. Credit: Network Rail

Managing director of England and Wales at Network Rail, Andy Thomas, said: "Our engineers and specialist extreme weather teams are monitoring track-side temperatures and vulnerable location.

"We will, if necessary, introduce temporary speed restrictions during the hottest part of the day to keep trains running, albeit more slowly than normal."

The need to prevent problems also means there will be fewer trains, but this is better than causing a bigger problem and being unable to run any services say Mr Thomas.

How did the UK deal with it in the past?

In the past, the UK coped with extreme heat by leaving gaps along the line. Credit: PA

In the past, the UK's main system of coping with extreme heat involved leaving gaps, known as expansion joints, along the line, allowing rails to slide past each other as they expand.

This meant trains were noisy and gave bumpy rides.

Nevertheless, jointed rail were only set up for the 40°C temperature range so even if they were still in place today, engineers would be having to take similar action.

What are we doing now to fix the problem?

Under the replacement system, to prevent buckling the track is "pre-stressed" or stretched and continuously welded.

Only when it gets unusually hot does the metal expand enough to pose a risk of rails pushing together and buckling.

Parts of the rail are painted white so they absorb less heat – and expand less.

Typically, a rail painted white is 5°C to 10°C cooler than one left unpainted.

The UK uses pre-stressed continuous welded tracks. Credit: PA

Why doesn't it happen in other countries?

Well truth be told they have big difficulties with it too and the solution entails costs that wouldn't be justified in the UK.

Japan, Austria, and other countries use "slab track" - where rails are laid on reinforced concrete slabs, holding them more rigidly - to varying extents.

It's also used in the section of the Eurotunnel which runs under the English Channel.

But Network rail estimates slab tracks costs approximately four times as much to install when compared to standard ballasted tracks.

Some countries who experience extremes in temperature that affect their railway adjust their rails between summer and winter, or have other measures in place to manage the effects over the long term.

However, Network Rail said it wouldn't be practical nor cost effective to implement the same measures permanently across Britain.