Two sidings join the main track of a railroad.
They meet together and lead on to a dead end. All this is shown in the
center of the illustration above. The dead end is long enough to hold
a car or an engine at a time.
Currently a blue car is located on the left siding and a green car -
on the right siding. An engine is located on the main track on the
midway between the two cars.
The objective of the challenge is to exchange the positions of the
cars and then return the engine to its initial position as is shown in
the lower right corner. And it has to be done with the smallest number
of couplings and uncouplings.