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Trains in Kent will be running a slower speeds on Thursday and there will be fewer services because of the hot weather.
Network Rail bosses say because extremely hot weather is forecast, speed restrictions will be introduced across the network.
Metal rails in direct sunshine can exceed 50°C, and so speed restrictions will be introduced on lines to reduce the chance of rails buckling in the heat.
A spokesman for Network Rail said: "We will be running significantly fewer services on Thursday, and many trains will be much busier than usual.
"Those trains that do run will take longer to complete their journeys - particularly on longer-distance routes.
"Our online journey planners will be updated (tomorrow) with the detail of which trains will be running.
"We strongly advise passengers to avoid travelling, or re-time your journey to when trains are less busy - which we’re expecting to be off-peak, before midday.
"If you’re planning to travel with us on Thursday, please allow extra time for your journey and check before you travel to make sure your train is running."
Passengers travelling in hot weather are advised to carry a bottle of water with them, and not to board a train if they feel unwell.
A normal timetable is expected to run on Friday as temperatures are expected to have fallen.