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
The pilot of a vintage jet which crashed at the Shoreham airshow killing 11 people is to be charged with manslaughter by gross negligence.
The MOD has confirmed the death of Corporal Jonathan Bayliss of the Royal Air Force Aerobatic Team (The Red Arrows).
Holly has the latest forecast for the east of the region.