Economy of London project is construction of 42 km (26 mi) of new tunnels connecting stations in central London including a branch to Canary Wharf in east London. It is Europe's biggest construction project with a £15 billion projected cost. An additional line, Crossrail 2, has been proposed. London is served by six international airports which are the world's busiest city airport system by passenger traffic. These are Heathrow, Gatwick, Stansted, Luton, London City, and Southend. There are a number of proposals for expanding airport capacity for London including expansion of London Heathrow Airport and expansion of Gatwick Airport. The principal argument in
London Biggin Hill Airport be called into question. However, in 1958 Biggin Hill ceased to be an operational RAF station, becoming the Officer and Aircrew Selection Centre for the RAF. Due to the impending closure of the nearby original London Airport at Croydon, from 1956 much of the civilian light aviation from Croydon relocated to Biggin and it became a joint civilian and military airport. Croydon closed completely in 1959, at which time Biggin Hill became a mainly civilian airport with only occasional military flying taking place. The base was also used by the following RAF Regiment squadrons: Towards the end of 1963, the