Claim: London's Underground is one of the few Railway electrification systems that utilizes four rails to solve the problem of electrolytic damage in return currents, using resistors to keep stray earth currents at manageable levels.
Evidence: The Underground uses a relatively uncommon four rail system of electrification. The chief advantage of the fourth-rail system is that, in tunnels with a metallic (usually cast-iron) lining, the return traction current does not leak into the lining causing electrolytic corrosion there or in adjacent utility mains. The London Underground is a large metro system operating across Greater London and beyond, commonly known as "the Tube".