Mules are the electric locomotives used to move and position ships as they go through the locks on the Panama canal.
I expect the use of the word "mule" comes from the days when boats were moved through smaller canals
by mule-power and horse-power.

The mule system has been used by the Panama canal since its beginning. In the early years they provided motive power as well
as controlling the ships. Nowadays, most ships use their own power to move forward, but the mules are used to control the
position of each ship, in spite of the advent of bow and stern thrusters on ships.

	Front view of a mule. They still use buffers rather than the standard coupling and drawbar gear that were standardised by American railways. This is one of the 3rd-generation mules: note the air-conditioner on top of the cab!
	Side view of mule
	The mules also function as people-carriers, as they have a railed platform at each end. Most of them have a plastic seat as well.
	Wheel-to-rail contact does not give sufficient friction for the mules to ascend the steep slopes between lock chambers, nor to be able to control the position of each ship. The mules can therefore operate on normal track or they can get traction from the rack. This type of railway is know as "cog-wheel railroad" in the USA, and as "rack railway" in the UK: one focussing on the rack, the other focussing on the pinion wheel which engages the rack. I suspect also that the mules "hook over" the rack, to minimize the risk of being overturned by the pull on the tow-rope.
	Tracks not used for towing do not need the rack (on right of picture) but on entering the rack section (on left) the mules have a transition section
	Even the return-tacks and sidings need the rack where there is a steep slope.
	This is a second-generation mule, on display at Miraflores.
	A first-generation mule at Miraflores. The towing lines are fed to the winding drum through the roof, rather than through pulleys low-down on the sides.