The sub-surface network is the collective term for the Circle, District, Hammersmith & City and Metropolitan Lines. These are the oldest of the Underground lines and operate larger trains, in larger tunnels.
The original Underground element of the sub-surface railway was engineered using a ‘cut and cover’ technique, meaning that trains in central areas run along shallow tunnels close to the surface. The legacy of this technique is railway channels that cut through some areas of London, diminishing public access and constraining potential development due to implications of drilling depths.
Historically, the Metropolitan Railway Company was responsible for the expansion of the many of the routes. The company developed a number of housing estates and actively promoted the benefits of living in “Metro-land”, thus driving the development of the London’s suburbs.
Some stations, such as Hillingdon (1923) were built specifically to serve company’s suburban developments. Charles W. Clark’s Arts and Crafts style stations at Croxley Green (1925) and Stanmore (1932) were designed with the intention to blend in with their surroundings. Many of these stations in the commuter belt have large car parks that represent key property assets for TfL.