I take your point, cascaderailroad, but you may have missed my description above of my real-world scenario. The rear helper is not coupled to the train, so there is no need to uncouple the helper (in the real world). Perhaps uncoupled rear helping did not happen in the U.S., but it certainly did in steam days in Australia and England. So my desire for the player driver of the rear helper to stay in his cab is not unrealistic. When at the top of the grade, the real-world driver of the rear helper simply eased off the throttle and dropped off the rear of the train he was assisting.
Trainz does not allow an unmodified rear helper loco to remain uncoupled (an edit could mask the helper loco's coupling so it did not couple to the train). Therefore if the player driver drives the rear helper, he effectively controls both the front and rear locos from his position at the rear of the train. The problem is not uncoupling the helper from the moving train at the top of the grade. Rather it is the behaviour of the train once the helper is uncoupled. In my experience, the train continues to move, but it has lost the capability to be under AI schedule control or player control. The train continues on out of control until it crashes. My testing is done in TANE SP1.