Temperatures across the South East are forecast to reach 37°C on Thursday and steel rail temperatures could rise to well over 50°C, according to Network Rail.
It has activated its ‘extreme weather action teams’ across the South East and passengers are advised to check before they travel.
A spokesman said: "To reduce the risk of buckling and to keep passengers safe, we have to introduce speed restrictions which means fewer trains can run and they’re likely to be much busier and take longer to complete their journeys.
"Our network is made of 20,000 miles of steel track, which absorbs heat easily and metal rails in direct sunshine can exceed 50°C.
"When steel becomes very hot it expands and if there is no room for the rail to expand further, this can cause the rail to buckle. If rails buckle, the line has to be closed for repair before trains can run again.
"In some locations, we may have to introduce speed restrictions during the hottest part of the day at vulnerable locations as slower trains apply lower forces on the track and reduce the likelihood of buckling."
Thameslink, Southeastern and Southern will be running a reduced service with speed restrictions in place across Sussex. Passengers are advised to carry a bottle of water with them and not board a train if you feel unwell.
Online journey planners have been updated with details of which trains will be running and if you’re planning to travel on Thursday 25 July, please allow extra time for your journey and check before you travel to make sure your train is running.
John Halsall, Southern region managing director for Network Rail, said: “Keeping passengers safe and moving are our top priorities during this heatwave. That’s why we sometimes have to put speed restriction on to prevent our rails, that can be over 20°C higher than air temperatures, from buckling which can derail a train and cause huge delays.”