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A new mega-train was unveiled today to ease overcrowding on busy London rail routes.

The 12-carriage Thameslink trains will be able to cary up to 1,750 people, the equivalent of 21 double-decker buses.

They will be 50 per cent longer than most of the rail firm’s trains.

Thameslink currently has four 12-carriage trains, with a capacity of around 1,100 passengers.

But it has ordered 55 more and they will carry even more passengers due to the design and fewer seats per carriage.

The new Siemens Class 700 Thameslink train, unveiled at Blackfriars station this morning, will also have electronic signs showing which carriages have more space to sit or stand in, as well as how the Tube is running.

Walkways between carriages will be “spacious” so passengers can move more easily through the train.

Wider doors and aisles are designed to make it easier to get on and off.

Adaptive climate-controlled air conditioning will adjust to the number of passengers in a bid to make travelling more comfortable.

Changes to seating, which will be two-by-two rather than two-by-three, will be welcomed by some passengers as there will be more space to stand at rush-hour.

Some commuters though are likely to be riled by the reduction of seats per carriage, though rail bosses stressed that there would still be more than on the eight-carriage trains as the new model is so much bigger.

The new trains will be running on the Brighton to Bedford route through central London within weeks.

Govia Thameslink Railway chief executive Charles Horton said: “These trains have been designed to meet the huge growth in passenger numbers that have soared 40 per cent in the past decade.

“Their wide doors, open interiors and state-of-the-art real-time passenger information systems will help us run a high frequency, high intensity service with over 80 per cent more seats across the heart of London in 2018.”

Rail minister Claire Perry added: “Projects like this show how our record £40 billion investment in the railways is delivering better journeys for passengers across the South East and beyond.”

The new fleet, which also includes 60 new more spacious eight-carriage trains, will run on routes to London from Sussex, Surrey, Kent, Hertfordshire, Bedfordshire and Cambridgeshire.

The trains will have fully accessible toilets for disabled passengers and those with pushchairs or needing baby changing facilities, as well as spaces for full-sized bikes in the off-peak, and storage for fold-up bikes at rush-hour.

With the major rebuilding of London Bridge station, the new trains will run through central London under automatic control every two to three minutes by end of 2018.