Extremely hot weather is forecast for Thursday, and speed restrictions will be introduced across the rail network, a Southeastern spokesman said.
The spokesman added that metal rails in direct sunshine can exceed 50°C, and Network Rail needs to introduce speed restrictions to reduce the chance of rails buckling in the heat.
Southeastern 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.
Online journey planners will be updated on Wednesday, July 24 with details of which trains will be running.
A Southeastern spokesman said: “We strongly advise passengers to avoid travelling, or re-time journeys to when trains are less busy.
“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 you feel unwell.
“We’re planning to run our normal timetable on Friday, July 26, when temperatures are forecast to have fallen.”
For more information about why high temperatures can cause problems for the railway, visit www.southeasternrailway.co.uk/summer-weather.