The deep-level tubes

The Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City lines are deep-level tubes, with smaller trains that run in two circular tunnels with a diameter of about 11 feet 8 inches (3.56 m), lined with cast-iron or precast concrete rings, which were bored using a tunnelling shield. It is these that were the tube lines, although since the 1950s the term "tube" has come to be used to refer to the whole London Underground system. Many of the central London deep-tube line stations, such as the Central and Piccadilly lines, are higher than the running lines to assist deceleration when arriving and acceleration when departing.^] The deep-tube lines generally have the exclusive use of a pair of tracks, except for the Piccadilly line, which shares track with the District line between Acton Town and North Ealing and with the Metropolitan line between Rayners Lane and Uxbridge, and the Bakerloo line, which shares track with London Overground services north of Queen's Park.