The project will include the acquisition of an additional 57 intermediate vehicles from Bombardier to strengthen the fleet of class 378 emus, which operate all services on the orbital suburban network and the so-called dc lines from Euston to Watford Junction. Delivery of the new vehicles will be completed by the end of 2015, boosting the capacity of each set from around 700 to 850 passengers.

The introduction of longer trains will require a number of infrastructure upgrades, including:

  • additional capacity at Silwood stabling sidings and Willesden depot
  • depot layout modifications at Willesden and New Cross Gate, and
  • platform extensions at some stations on the North and East London lines, with associated signalling and electrification works.

Subject to planning approval, enabling works will begin next month with the upgrading of Silwood sidings.

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.