I worked for the Milwaukee in the late 60s, and we used hands by day and lanterns by night to talk to each other in the yards. We didn’t call them “signals” though. We called them “signs.” One of the joys of day work was the use of hand signs rather than lugging the lantern around all night. As others have mentioned, there were signs for come to me, move away, easy, cut off, go into the clear, stretch the joint, cut in the air, kick, car lengths, and all the numbers to designate tracks. The C&NW had different signs for most of the numbers, but most of their other signs were the same as the Milwaukee.
It is interesting to note how skeptical people about the reliability of hand signs in these days of the radios. If we were working over a long distance, we would make the signs as large as possible. If visibility were restricted by a curve, signs would be passed between several people. The rule was that if anybody went out of sight, the person who lost sight, must pass a stop sign to the next person. If the engineer lost sight of the person passing him signs, he was required to stop. The body language of these signs was remarkably unmistakable, even at considerable distance. The stop sign in particular, was brilliant body language. It just looked like “give it up.” There were also signs to tell somebody about a hotbox or sticking brakes. The hotbox sign was plugging your nose. If you plug your nose and pat your head, it means hotbox near the head end.
When calling signs to the engineer, across the cab, we never used the word, “stop” because there was a tradition that believed that giving such a direct order to the engineer would be disrespectful. So in instead of “stop” we said “that’ll do.”