Britain faces its hottest day on record today, with temperatures of up to 39°C (102.2°F) forecast in parts of southern England.
But the hot weather is disrupting train services across the country, amid fears that it could cause rail tracks to buckle.
Great Western has cancelled trains between London, Cardiff and Swansea, and speed restrictions have been put in place to protect tracks.
Train companies operating services into London from Kent, Sussex, Cambridge and other commuter-belt counties are also running fewer trains and imposing speed restrictions.
Meanwhile, Transport for London is running fewer trains than normal on some London Overground and TfL Rail services.
Network Rail explained that, when Britain enjoys a summer heatwave, rails in direct sunshine can be as much as 20°C hotter than air temperature.
"Because rails are made from steel, they expand as they get hotter, and can start to curve – known as 'buckling'," it said.
"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."
If the track does buckle, the line must be closed and the track repaired before services can resume, causing considerable disruption.
Usually, these repairs can't be done until the temperature of the rails has dropped.
So if a section of track is judged to be at risk, Network Rail introduces local speed restrictions. Slower trains exert lower forces on the track, and reduce the chance of buckling.
This isn't the only precaution rail companies take to protect the tracks.
For a start, they work with specialist weather forecasters to work out which areas might be most affected so they can take action.
There are also mini weather-stations and probes installed track-side to monitor local conditions more closely.
In some cases, staff even paint certain parts of the rail white so that they absorb less heat, reducing the risk of expansion. This can make the rails 5-10°C cooler.
Train delays or not, it's also important for passengers to consider their own health and safety during hot weather.
Remember to carry a water bottle, to stay hydrates, and if you feel unwell, get off at the next stop where a member of staff can help you.
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