In pre computer days, at the most basic, the engineer had 8 run positions, as the throttle advanced the prime mover advanced to the next RPM step, in turn generating more electrical power for traction motors and away we went. I know there is a governor involved but I am ignoring its function for these cases.
With new computerized diesels and AC traction, the engineer still has 8 run notches, but does that prime mover still advance to the next set RPM like before, or does the computer(s) regulate how fast to turn, generating just enough power for traction motors to move the load for that notch, thereby saving fuel, emissions, wear and tear. So the prime mover might not sound the same per notch depending on the train load pulled.
