Randy Stahl
The only hand signal from anyone but my crew that I would acknowledge would be a frantic motion from anyone, which means stop.
There was a story in Trains (or maybe "Classic Trains") some time back in which it was related that something was going awry and the man on the ground was giving a frantic "stop" signal, with both hands/arms swinging in the prescribe direction ("lower quadrant").
The engineer saw the signal and called across the cab to the rest of the head-end crew, "Hang on boys, he's trying to fly!"
I've heard of folks using fist pumps to indicate a car count, and some of ours will flash finger counts (ie, flash both hands four times for forty feet). I figure that if the engineer can see me (this usually involves recoupling the locomotive to the consist after a runaround), he knows about where he is. It's the last 10-15 feet that can be deceiving.
I was running once when we encountered a traffic cone on the end of a tie. It probably got dropped/knocked from the overpass we were going under, and wasn't any of prescribed methods for warnings, but we stopped, then proceeded at restricted speed anyhow, just to be safe.
