A riddle/puzzle I can't figure out =(?!

Two sidings join the main track of a railroad!. They meet together and lead on to a dead end!. 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!. An engine is located on the main traack 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!. How many move must be done!?

Can you please tell me the answer and why!? I don't really understand the question to answer it!. This is not homework (I'm too old for that) it's a puzzle in a newspaper I can't figure out!!Www@Enter-QA@Com

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