London Underground at +420 V, giving a potential difference of 630 V. On the sections of line shared with mainline trains, such as the District line from East Putney to Wimbledon and Gunnersbury to Richmond, and the Bakerloo line north of Queen's Park, the centre rail is bonded to the running rails. The average speed on the Underground is 20.5 mph. Outside the tunnels of central London, many lines' trains tend to travel at over 40 mph in the suburban and countryside areas. The Metropolitan line can reach speeds of 62 mph. The London Underground was used by 1.379 billion passengers in
South London line South London line The South London line is an Inner London part of the London Overground rail network. The line is run together with the East London line to provide direct services between and . It consists of eight stations, one of which marks the crossover into the East London line network and runs . Most of the line is on high viaduct over other transport infrastructure. Interchanges with the London Underground are at and the closest on its London Overground extension is . The line is in Travelcard Zone 2. From the early 20th century until 2012 a shorter