FirstGroup and Hong Kong-based MTR said the 750 new carriages on the South Western network will boost capacity by 46% and cost Â£895m.
The Aventra trains will operate on the Reading, Windsor and London Suburban routes out of London Waterloo.
They will start to come into service from mid 2019.
The new carriages, in five and 10-car formations, will operating by December 2020, and will feature free Wi-Fi, at-seat charging points, information screens, air conditioning, wide gangways and wide doors, FirstGroup said.
The company also aims to bring 90 refurbished carriages onto the Portsmouth to London fast services by December 2018.
FirstGroup and MTR are due to take over the services across south west England, currently run by Stagecoach, on 20 August.
The fleet of new trains will be manufactured by Bombardier Transportation in Derby.
The new operators of one of the country's busiest rail franchises has announced it has placed an order for 90 new trains.