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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Tuesday evening's weather for the west of the region
Tuesday evening's weather for the east of the region
A man has been cleared of smuggling 35 immigrants in to the UK in a container. One immigrant, Meet Singh Kapoor, died during the journey.