The intermediate vehicles will be assembled at Bombardier's Derby plant and are due for delivery between the end of next year and late-2015, increasing the capacity of each class 378 set from around 700 to 850 passengers. The vehicles will be maintained by Bombardier under the maintenance contract for the existing fleet.

The introduction of longer trains will require a number of infrastructure upgrades, including additional depot capacity and platform extensions at some stations on the North and East London lines, together with associated electrification and signalling works.

Since TfL took over the former Silverlink Metro network in 2007, demand has increased by 160%, and if the East London Line is included this figure rises to 280%. The Overground carried 120 million passengers last year, and the recently-opened extension from Surrey Quays to Clapham Junction is expected to boost ridership by a further 12 million passengers per year.

London's population is expected to increase by 810,000 between 2007 and 2021, and employment in the city is forecast to grow by 670,000 jobs over the same period. TfL expects Overground ridership to increase by 400% from 33 million passengers per year to 165 million between 2007 and 2021.