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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Tens of thousands of rail passengers are facing mass train cancelations every day at Southern because of ongoing sickness by guards.
Wednesday afternoon's weather for the east of the region
Wednesday afternoon's weather for the west of the region