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
Attacks on the London Underground On 9 December, a van bomb partially detonated at the car park of Woodside Park station. No-one was injured but it caused evacuations and disruptions. A device exploded at an underground passageway at South Kensington station on 3 February 1993. On 20 December, a device exploded in a litter bin in Northfields station. On 21 December, coded bomb warnings from the IRA resulted in a paralysed London Underground system, as tens of thousands were evacuated from 100 tube stations during the morning rush hour. In 2005 two groups of Islamist extremists attacked a number of underground lines and bus routes