A 12-carriage train has been tested on the Thameslink route through London to trial the service before expanding it in the UK.
First Capital Connect and Network Rail organised the empty test train service between Bedford to Brighton on 3 October this year to check factors such as stepping distance checks, levelling and heights of the train and newly-extended platforms.
Network Rail said it has partially completed a programme to extend platforms at 12 stations north of London on the Thameslink route and is upgrading power supplies for the longer trains.
The entire service will be remodelled to accommodate longer trains as part of the final phase of the government's £5.5bn ($8.7bn) Thameslink Programme, involving the remodelling of Farringdon and Blackfriars stations in central London.
The first 12-carriage trains will enter service in December 2011, with four trains each running in the morning and evenings between Bedford and Brighton, with further trains to come in the future.