High temperatures have been disrupting trains between London and the West with speeds being cut over fears of rails buckling.
Great Western Railway (GWR) has amended some services from 13:30 to 19:00 BST because of the heat.
High-speed trains between Paddington and Oxford have been cancelled.
Network Rail said modern rails were far more resistant to buckling but "on a day this hot" speeds needed to be restricted for safety.
Temperatures are set to exceed 30C (86F) in London and Thames Valley, the Met Office said.
Passengers to Oxford have been advised to either take stopping services or travel to Didcot for connecting services, and are advised to check their specific journey before they travel.
Trains to and from Paddington and Bedwyn, in Wiltshire, will operate between Bedwyn and Reading only.
And Paddington to Cheltenham Spa services will only operate between Swindon and Cheltenham Spa.
To prevent buckling, small gaps are left between sections of steel rails to allow them to expand and contract as temperatures change normally throughout the year.
However, in extreme hot spells when the air temperature reaches 30C the rails can reach 50C (122F) meaning they are at risk of buckling, resulting in speed restrictions.
Tracks in some overseas countries are often set up for constant higher temperatures.