If I understand correctly: You are saying that the engineers of Japanese locomotives had to shout to each other to request what they needed, and so they began pointing, too? Thus, the person to whom they spoke could rely on either their ears or eyes to understand?

And—I think you are also saying this—the engineer who made the request was less likely to forget or otherwise "mess up" the move he'd just requested a minute earlier, since he'd requested it with both his voice and his hands, so his mind was less likely to "wander" or daydream?