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.
More top news
Thursday morning's weather for the east of the region
Thursday morning's weather for the west of the region
South East Coast Ambulance Service will be put into Special Measures, following concerns raised in a Care Quality Commission inspection.