Dirk and Earl,
We never used them in the yard either. Too much racket. I only used them out on the road when the Brakeman or Conductor was using hand signals, or when I was around a curve and could not see either employee when they were giving me orders by radio. When you're switching or setting out or picking up, it's easier to grab a valve handle and give it a few yanks than to hassle with a mic cord.
In the yard we never used them unless there was a vehicle at one of the internal yard crossings or there were carmen or sectionmen working near bye who needed to be alerted to our presence.
Yes, there are signals listed in the General Code, the Consolidated Code before it and the Railroad Specific rule books before them, but the only signals that were used regularly were the road crossing signals. To the best of my knowledge there is no law governing the use of anything other than the road crossing or a red flag during a test.
Rick