A train company has ordered 48 new electric carriages as part of a package designed to end "cattle truck" overcrowding at rush hour.

Southern has signed a £58 million contract for the Electrostar trains which will run on its routes in Sussex and south London.

The carriages will replace the class 319 trains which have been operating for 15 to 20 years. Those older carriages will be signed over to First Capital Connect for use on the Thameslink route, increasing the number of seats on their services from Bedford to Brighton through central London by 8,300 at peak times.

Southern has signed a contract with train leasing company Porterbrook and train manufacturer Bombardier.

The deal follows an announcement last month by the Department for Transport of a series of moves to improve packed services.

Other measures included the surprise decision to extend the Gatwick Express, which runs from London to Gatwick, to Brighton at peak times to increase the number of services between the capital and the seaside city.

Chris Burchell, managing director of Southern, said: "We are delighted to have closed this deal so quickly and efficiently. This has secured a tangible move towards helping with the continued growth in passengers across the South East.

"These new trains will make a real contribution in meeting the future needs of the railways, as it becomes an ever more popular mode of transport."

Shelley Atlas, the chairwoman of passenger campaign group Brighton Line Commuters, welcomed the move, saying: "Southern trains will be improved. That has to be a good thing."

She said the Electrostar trains had fewer seats per carriage, but were more comfortable.

The 319 trains can be used on the Thameslink route because they operate using overhead power lines. The first 17 are expected to be running by the end of 2007 and the rest should be in place by the end of next year.

The signing of the contract is one of the first steps in the rail industry's challenge to deliver 1,000 new carriages over the next seven years.

A total of 12,000 extra seats will be provided in Sussex, the result of a three-year review of one of the country's busiest commuter routes.

Trains from Brighton to London carry 25 per cent more passengers on the East Croydon to London stretch than they should.

But Gatwick Express trains are, on average, only 43 per cent full at peak times, which led to plans to scrap the airport link.

T Department for Transport said the service would be retained as a non-stop route and would keep its 15-minute frequency. At peak times, the service will run beyond Gatwick to Brighton, doubling the number of express trains between Brighton and London Victoria in busy periods. Some services will call at intermediate stations.

Unused coaches will be reconditioned to provide extra capacity. The changes will provide more than 3,700 seats on trains into and out of London at the busiest times.