I believe the Daylight excoursion crews will all call a signal, no matter who sees it first, to amek sure thsat neither the fireman, engineer, conductor, or pilot are dozing at the “wheel”. Depending on the engine though, the engineer could say “fluebenstuffel” and the fireman couldn’t hear him anyway.
If your “americanizing” Highball would be most suited.
But the engineer wouldnt have to say anything, he takes the commands to move. His voice is the whistle.
Thye may talk in the cab to the fireman if all clear, or if a signal can’t be seen by the engineer on a curve the fireman will repeat the signal indication.