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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Mostly dry with sunny spells and a fresh south-westerly breeze.
Cloud and sunny spells with a fresh breeze this afternoon.
Saturday morning's weather for the east of the region