The new trains, expected to cost around £1.4bn ($2.8bn), will substantially increase passenger capacity on the route, which is to undergo a £3.5bn upgrade that will increase the number of trains and open up more northern destinations.

The size of the order is not yet finalised, but it will be between 900 and 1,300 new carriages. When the upgrade is completed, with older trains redeployed elsewhere, there will be around 380 more carriages than in the existing fleet, which is currently severely overcrowded because of capacity constraints.

Ruth Kelly, transport secretary, will say on Wednesday that the extra carriages will provide 14,500 more seats on some of the most overcrowded commuter routes into London.

The new vehicles are in addition to an order for 44 new carriages placed by the DfT in March with Bombardier Transportation, the world’s largest trainmaker. Those carriages are intended to help Southern, the train operator at the route’s southern end, to provide services over part of the route while the upgrade programme is under way.

The northern part of the route – which will link the existing cross-London service to lines from Kings Cross – will be completed in time for the 2012 Olympics. Work on the southern part of the route, where trains are currently held up by a stretch of single track near London Bridge station, will then go ahead after 2012.

As one of the largest single train orders ever placed in the UK, the contract will be keenly contested by big trainmakers.

The fleet will need electrical equipment to deal with both the low-voltage third rail south of the River Thames and the high-voltage overhead lines north of it.

The government promised in last year’s rail white paper to provide another 256 carriages for the Thameslink route by the middle of the next decade.