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
Central line (London Underground) and realigned and the stations lengthened, but it proved impossible to modify Wood Lane station to take 8-car trains and a new station at White City was authorised in 1938. The line was converted to the London Underground four-rail electrification system in 1940. The positive outer rail is higher than on other lines, because even after reconstruction work the tunnels are slightly smaller. Most of the tunnels for the extensions to the east of London had been built by 1940, but work slowed due to the outbreak of the Second World War until eventually suspended in June. The unused tunnels