With a canal lock that has only one chamber, it's probably faster for southbound and northbound ships to alternate through the lock. But where there is a staircase of locks (three at Gatun, two at Miraflores) you can get more ships through in a given time by using
the convoy system: a convoy one way followed by a convoy the other way. So the Panama canal now  has ships moving from the sea to Gatun Lake in the mornings, and from the lake to the sea later in the day. But this means that in the mornings, all the mules would accumulate at the Lake end of each lock, and in the afternoons, at the sea-level ends of the locks, so there has to be
provision for returning the mules to their starting points.

To do this, each flight of locks has a return track set back from the towing-track which is at the water's edge.  The return track
does not need the cog-wheel and rack, as the mule does not have to provide any towing force. But there's another complication:
in the centre island, between the two parallel flights of locks at each location, there is only one return track for both towing tracks. And since the mule picks up its electrical power from a conduit, and since the conduit is at one side of the track, each mules must
be turned to the correct orientation before it goes onto the return track and before it is positioned on the towing track.

These pictures, taken at Gatun, show mules being positioned after they have returned to the start point.

	Only one mule is parked on the track adjacent to the Statendam, so others are being sent down the return track to assist. They will be turned on the turntable and put on the adjacent track. The first one is approaching the first control panel, set on a post so that the operator does not even have to leave his air-conditioned cab. He will make the table rotate until its track is in line with the return track.
	The mule parked on the "adjacent" track has moved forward to service the Statendam. The first return mule is moving onto the turntable, and the second return mule is approaching the turntable control
	The first return mule is on the turntable, and the operator is about to lean out and punch instructions into the second control station, mounted on a pole actually on the turntable
	The operator pushes the buttons . . .
	and the table begins to turn.
	Table is now halfway around. More ships approaching in the distance.
	Table almost completely around: when the turn is complete, the mule can move off the table and allow it to be used by the next mule.
	The operator of the next mule (158) directs the turntable to rotate so that the tracks are aligned to allow his machine to run forward onto the table.
	Table has turned, and mule 158 can roll onto it
	The table, with mule 158 on it, is rotating. When the tracks line up, mule 158 can move forward onto the track adjacent to the water.
	The central return-track (with no rack-rail) for mules cannot be used at Pedro Miguel as there is a crane parked there, so mules have to return to the start point in convoy, between ships.
	Aerial view of a turntable, at Gatun.