I'll probably get the car lengths wrong here, but some of the backing signals are these. When a train is backing, you should see the brakeman or conductor with his arms wide over his head, rotating his hands. This means the cars are more than 1 car length apart.
When he stops rotating his hands the cars are within one car length, and as the cars get close enough together the brakeman will move his hands together to indicate the actual distance between cars. Then, as Erik mentioned, he'll put fist into hand to indicate contact.
If the cars fail to couple, I believe he'll wave his hands in a sort of x pattern in front of him indicating the engineer should pull forward and try again.
Before he moves between the cars, either to reset the coupler or to connect the air hose, he'll swing his arm in an arch high to low to indicate he wants to go between the cars. If he's smart, he'll wait until the engine acknowledges with a single toot - apply brakes - before moving between the cars.
And, of course if he's a she, she'll do the same thing only with different pronouns
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Don