Thousands of passengers will be able to use free wi-fi on their train journeys.
South West Trains said the service will be available on its Class 444 Desiro electric trains on its main line route from London Waterloo to Weymouth and Portsmouth.
The rail operator said it has made the move to help customers make more use of the time they spend on trains.
The news has been welcomed by a passenger group.
More than 30,000 people travel along the main line every weekday.
Bruce Williamson, from Railfuture, which campaigns for better rail services for passengers and freight, welcomed the news.
He said: "Wi-fi is starting to become a 'must-have' for public transport, because so many people are permanently plugged in to their phones and tablets.
"It is a very sensible and welcome move and I think that before long we will reach a stage where there is blanket coverage on every train."
However, Mr Williamson warned train companies should not lose sight of other passenger complaints - "primarily over-crowding and the level of fares."
The wi-fi has been installed through a partnership between South West Trains-Network Rail Alliance and Nomad Digital.