Siemens said the deal, struck with the Department for Transport, was the biggest it had secured in Britain and the largest for its rolling stock business.
The agreement, in partnership with industrial consortium Cross London trains, is for the Thameslink commuter railway route which connects Bedford with Brighton, cutting across London.
"Introducing a high capacity, high frequency service of longer trains, extended platforms and new stations, the project is regarded as one of the largest rail infrastructure projects in the UK," Siemens said in a statement.
The trains will be manufactured in the western city of Krefeld, with the first to enter service in 2016, Siemens said.
It said that with this contract, it will have supplied nearly 3,000 rail vehicles to Britain.