The mules run on broad gauged tracks. Traction is by electric power, supplied through a third rail laid below surface level on the land side. Each mule has a powerful winch, operated by the driver; these are used to take two cables in or play them out in order to keep the ship centered in the lock while moving it from chamber to chamber. With as little as 2 feet of clearance on each side of a ship, considerable skill is required on the part of the operators.