Speed restrictions are being introduced on Britain's railways amid concern that rails will buckle as temperatures soar.
Network Rail's extreme weather action teams are being activated across the country to monitor "vulnerable locations".
Monday was the hottest day of the year so far as temperatures reached almost 30C in some parts of the country.
Forecasters expect the next few days to be even hotter.
Andy Thomas, managing director of England and Wales at Network Rail, said: "On very sunny days, rails in direct sunshine can be as much as 20C above air temperature, causing the steel to expand markedly, and could - if not carefully monitored and action taken - buckle, causing travel disruption.
"Our engineers and specialist extreme weather teams are monitoring track-side temperatures at vulnerable locations and will, if necessary, introduce temporary speed restrictions during the hottest part of the day to keep trains running, albeit more slowly than normal."
Some sections of rails are painted white so they absorb less heat.
Slower trains exert lower forces on the track, reducing the likelihood of buckling.