Several fast trains from London Paddington have been cancelled by First Great Western today, after Network Rail imposed speed restrictions to prevent track from buckling in the heat.
Six services out of 1,520 running today have been affected so far.
A spokesman said: "First Great Western is advising passengers that due to the anticipated hot weather Network Rail is to impose a speed restriction in the London Thames Valley area to protect track points on Tuesday 30 June.
"As a result there will be no fast trains between between London Paddington and Bourne End or Henley-on-Thames from 12:00 until 20:00."
Network has told train companies to slow down at vulnerable locations as tracks could buckle.
Cooler temperatures are forecast after thunderstorms and heavy downpours hit parts of Britain after the hottest July day on record.
After a sweltering day, where temperatures reached a record high for July, the weather turned.
Paddling pools, ice creams and sun hats were among just some of the ways people kept cool during the hottest July day ever recorded.