The 290 HP locomotives are used to keep ships in the proper relation to the locks structures, and not to pull the ships in and out the locks as many people still think. They are essential for a safe and expeditious transit, and run on tow tracks along the lock walls from one end to the other. The locomotives are important to avoid too much paint damage on the ships which costs a lot of money for maintenance.

A new generation of locomotives has now been incorporated into the Panama Canal. Each locomotive weighs 50 tons, operates with two 290 HP traction units and has a towing capacity of 312 kilonewtons at 5 kilometers per hour, and of 178 kilonewtons at 8 kilometers per hour. They can move the same load at 3 and 5 kilometers per hour, respectively. Their maximum return speed is 16 kilometers per hour. They are lighter, stronger, and faster than the previously used models. Reduction in lockage time is among the benefits of their new design.

The original locomotives used in the Canal, or "mules" as they were also called, were built by General Electric. They weighed 43 metric tons and could pull 111 kilonewtons at a towing speed of up to 3.2 kilometers per hour. Their return speed was 8 kilometers maximum. As of 1964, they have been replaced with Mitsubishi locomotives that weigh 55 tons and tow up to 311 kilonewtons at their maximum towing speed of 4.8 kilometers per hour. They use two windlasses, and have a return speed of 14.4 kilometers.

In 1997, the Mitsubishi Corporation was awarded the contract for the manufacture of new locomotives. The first eight prototypes were delivered in August 1999, at a cost of $2.3 million each. After a 6-month test period, they began operating at Miraflores Locks.

The second group arrived in August 2001, at a cost of $1.9 million per unit. The fleet has now increased to 100 units, and the full replacement of the fleet with this new type of locomotives is now under way.

The size of the locomotive fleet has grown over the years in order to cope with the increase in number and size of the vessels transiting the Canal. Most of the ships transiting the locks used the assistance of four locomotives. Today, because of their size, some ships require up to eight of the new, stronger locomotives.