It follows a campaign by commuters for a later rush hour service in the evening to help ease overcrowding.
Other Thames Valley services will have more carriages with first class compartments converted for use by standard ticket holders. There are also extra trains between Swindon and Westbury.
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Philippa has the weather for the bank holiday weekend in the east
Philippa has the weather for the bank holiday weekend in the west
The weekend will see plenty of sunshine across most of the UK.