First Great Western has amended some services from midday to 20:00 BST because of the expected intense heat.
There will be no fast trains between London Paddington and Bourne End in Buckinghamshire or Henley-on-Thames in Oxfordshire.
Network Rail said if the track buckled the line must be closed for repairs "causing considerable disruption".
Trains from London Paddington will terminate at Twyford for a connection with a branch line service to Henley-on-Thames.
Passengers travelling to Bourne End will see their services from Paddington terminate at Maidenhead and will also have to complete their journey on a branch line train.
Temperatures are set to hit 30C (86F) in parts of the Thames Valley, the Met Office said.
Hot weather is set to disrupt trains in the Thames Valley area with speeds being cut over fears of rails buckling.