From the outset, it was considered an important safety feature that ships be guided through the Panama Canal lock chambers by electric locomotives, known as mulas (mules, named after the animals traditionally used to cross the isthmus of Panama), running on the lock walls. These mules are used for side-to-side and braking control in the locks, which are narrow relative to modern-day ships. Forward motion into and through the locks is actually provided by the ship’s engines and not the mules. A ship approaching the locks first pulls up to the guide wall, which is an extension of the center wall of the locks, where it is taken under control by the mules on the wall before proceeding into the lock. As it moves forward, additional lines are taken to mules on the other wall. With large ships, there are two mules on each side at the bow, and two each side at the stern—eight in total, allowing for precise control of the ship.
The mules themselves run on rack tracks with broad gauge, 5 ft, to which they are geared. 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 pay them out in order to keep the ship centered in the lock while moving it from chamber to chamber.
The old canal locks can barely fit the huge container ships – they often come too close to the edge with hardly room to spare. With as little as 2 ft (60 cm) of clearance on each side of a ship, considerable skill is required on the part of the mule operators.