The main line has a single diameter tube with two tracks. Just before stations, tunnels diverge into two single-track diameter tunnels leading to two parallel diameter station tunnels. The main tube is at a depth of around . Stations are at a much shallower depth, with a 1-in-20 gradient into the stations. The gradients assist in slowing the trains when approaching stations, and accelerating them away. There is also less distance to lift mail from the stations to the surface. At Oxford Circus the tunnel runs close to the Bakerloo line tunnel of the London Underground.