Thameslink should have an entirely new fleet by 2015
A busy commuter route is to benefit from 1,100 carriages in a £1.4bn deal.
The new, energy-efficient trains will provide an additional 14,500 seats on the Thameslink route.
The new trains will run from 2012 between London's St Pancras and Blackfriars stations and on services on the Midland and East Coast lines.
They will also run via London Bridge and Elephant and Castle to destinations on the Brighton line and on other routes in Kent, Surrey and Sussex.
Then deal, announced by Transport Secretary Ruth Kelly, will mean the current 720-carriage Thameslink fleet increasing by 380 carriages.
A contract for the new carriages is expected to be awarded in summer 2009, with the first train in service by 2012.
Thameslink should have an entirely new fleet by 2015, with the old carriages being switched to other routes.
To make effective use of the new trains, the platforms at Blackfriars station will be extended to make it the first station to span the width of the River Thames.
There will be additional improvements at Farringdon and London Bridge stations to enable the increased services.
By December 2015 the current bottlenecks at London Bridge will be eased to enable 18 Thameslink trains per hour to serve this station.
Another six trains running every hour between Blackfriars and St Pancras International via Elephant & Castle will increase capacity of trains through central London to 24 trains an hour.
Ms Kelly said: "This is a major step forward in our commitment to improve passengers' journeys and increase capacity on the railway."
Anthony Smith, of customer watchdog Passenger Focus, said: "Working in conjunction with the government and London TravelWatch, we are delighted to be helping ensure that passengers' views are at the heart of decision making about the design of the next generation of trains to run on these busy routes."