Rail operator First Group is to take over the franchise currently run by Thames Trains from April 2004.
First Group will integrate the franchise with Great Western
The Strategic Rail Authority (SRA) picked the company as preferred bidder last November, and selected a "package of benefits" put forward by the company.
First Group's proposal included improving integration of services with its current Great Western routes.
The current franchise runs from Paddington to Wales and the West of England.
The Thames franchise covers routes to Berkshire, Oxfordshire and Bristol.
The company says it will deliver improved punctuality and run high-speed trains to replace Thames' current fleet.
The SRA also announced that First Group would introduce new electric local services along the Heathrow Express line.
Moir Lockhead, First Group's chief executive, said: "I am confident we can provide Thames' customers with a train service that will combine improved punctuality and enhanced customer service.
"We will also be able to bring benefits to First Great Western customers in the South West and South Wales."
The first set of integrated timetables will start from December 2004.
The franchise will run for two years, after which the SRA will combine First Great Western, Thames and Wessex Train's routes into a "Great Western" franchise.