Longer trains set to ease congestion on Thameslink line
The first 12-carriage trains have been rolled out on the Thameslink route between Bedford and Brighton.
First Capital Connect are running the 50% longer trains on six services a day from Bedford, creating almost 1,500 extra seats.
More trains will be increased from eight cars to 12 when a new fleet is delivered in 2015.
Four peak four-carriage trains have also been doubled in length creating a further 1,200 seats.
Network Rail has upgraded power supplies and signalling systems and lengthened 41 platforms at 12 stations on the Midland main line north of London to accommodate the new trains which are nearly 270 yards (250m) long.
The work was a key element of the Thameslink Programme, a £6bn government-funded initiative to ease congestion on the Bedford to Brighton route.
Customer service director Keith Jipps said: "We're working hard to improve services for customers and have targeted the extra capacity on those trains that need it most.
"The Thameslink Programme has imposed some tough times on our customers with lots of track closures for engineering work but with the start of the new 12-carriage services we can finally see improvements coming on line."
The 12-carriage trains will initially run between St Albans and St Pancras International.
Longer trains will be able to call at Radlett, Elstree and Borehamwood, Mill Hill Broadway and West Hampstead from 2018 when the final Thameslink Programme timetable is introduced.
Platforms have not been extended at Kentish Town because of road bridges at each end which cannot be moved, or at Cricklewood and Hendon because of "insufficient demand" to warrant running longer trains.