Are four-legged mules and not tractors still used on the Panama Canal for barges less than 500 tons?

Answer 1

No. There are what are called handline transits where small vessels are controlled by the use of hand lines between the vessel and the canal walls. A 500-ton vessel, or one even a fraction of that weight, could do severe damage to the lock gates if it got loose, so the diesel electric "mules" are used on anything of significant mass to maneuver is safely into and out of the locks. Though mules, the four-legged variety, were used along early canals in Europe and the U.S. to move barges, they were never used in the Panama Canal. From the onset, the protection of the lock gates mandated the use of the powerful machines we see there today. GE built the first ones, and Mitsubishi builds the current models in use there. A link is provided to the Wikipedia article on the Panama Canal locks. You can check facts there, and can fill out your knowledge of the Panama Canal as well.