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
More than 300 Red Kites are visiting Reading every day - to feast on meat left out for them by locals
Wednesday's weather for the east of the region
Wednesday's weather for the west of the region