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 Underground S Stock (ex-Metropolitan Railway) used with two 1925-built prototype driving motors. In 1925, two driving motor cars were built as prototypes for what would later become the T stock. In the London Passenger Transport Board numbering scheme, they became 2598 and 2599. After the trailer cars were withdrawn from service in the 1940s, the two driving motor cars remained in service on the East London Line, being used with a 1923-built control trailer. These three cars were withdrawn in 1950. London Underground S Stock (ex-Metropolitan Railway) S stock is the designation given to one train of eight cars by London Underground in the 1930s.