Claim: London's Underground is one of the few Railway electrification systems that utilizes four rails, which doesn't solve the problem of electrolytic damage in return currents as the system has no way keep stray earth currents at manageable levels.
Evidence: The London Underground (also known simply as the Underground, or by its nickname the Tube) is a rapid transit system serving Greater London and some parts of the adjacent counties of Buckinghamshire, Essex and Hertfordshire in the United Kingdom. These lines have the exclusive use of a pair of tracks, except for the Uxbridge branch of the Piccadilly line, which shares track with the District line between Acton Town and Hanger Lane Junction and with the Metropolitan line between Rayners Lane and Uxbridge; and the Bakerloo line, which shares track with London Overground's Watford DC Line for its aboveground section north of Queen's Park.