I've seen that gesture used on the ground to show how close cars were coming to be coupled.
The brakeman with arms extended would bring both outstetched hands toward each other as the cars were moving to show the relative gap closure, until he grasped both hands to indicate the coulping was achieved.
Helpful when pushing cars together that were a few car lengths away from the locomotive.
The brakeman could also use that gesture in reverse (hands together then arms spread out), to signal the engneer to pull away as the coupler pin has been lifted.
I'm not sure why this appears on top of a box car unlss there is another meaning.
It's not the best place to stand and signal a "go ahead".