Are you asking about today, or "back in the day"? Today, radio communication does the job nicely.
As an engineer, I can tell you that a good crew can communicate VERY nuanced information by hand signals. When a conductor who knows what he is doing is giving me hand signals, I can fairly easily spot a train to within a few inches of what he wants. All that is needed is to be able to see him clearly. I cannot believe that it would have been any harder back then - and it might have been easier, with smaller trains and much better control over the throttle.
Kenneth Rickman
Researching and modeling the Danville & Western Ry.
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