Switch the Cars

after Sam Loyd

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 illustration on the left. 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. And it has to be done with the smallest number of couplings and uncouplings.