There is another factor which enters into this - the mental end. When you are working the ground you are constantly planning moves, watching for hazards, calculating how far to the hitch, and a dozen other things. When you are running an engine you are constantly evaluating your speed, need more brake? need more throttle? watching for hazards, did some other job take the switches away from us, anticipating what you need to do next. Both jobs require full mental attention.
Now when you put both tasks on the same person at the same time you geometrically multiply the chances of something going wrong. Add in the possible malfunctions of the box as Steel Wheels describes and you are really putting mental overload into play - too much information. (Remember some guys can't chew gum and walk at the same time).
On a nice quiet industrial spur moving a few cars they would be OK in my opinion. But in a congested yard with a lot of work and other jobs - no thanks.
Now when you put both tasks on the same person at the same time you geometrically multiply the chances of something going wrong. Add in the possible malfunctions of the box as Steel Wheels describes and you are really putting mental overload into play - too much information. (Remember some guys can't chew gum and walk at the same time).
On a nice quiet industrial spur moving a few cars they would be OK in my opinion. But in a congested yard with a lot of work and other jobs - no thanks.