In regard to Fred's comment regarding restrictions on helpers on the rear of freight trains, I can add that in 1965 the PRR had a special instruction which pertained to helpers on the rear of freight trains.
At that time the rule was any freight train where the diesel horsepower on the pushing locomotives exceeded 3600 diesel horsepower, the cabin car crew had to ride the helping locomotives. This resulted in having to stop the train to transfer the rear end crew back to the cabin and cut the helpers off.
This was a State of Pennsylvania PUC ruling and not anything that came from the federal ICC. I have seen PRR steel N5 class cabin cars that were buckled due to too much power on the rear end of a large 140 car freight train and the buffing effect when traveling uphill on back to back reverse curves.
7 Boundary Road, West Branch
Newark, Delaware, 19711-7479
(retired train dispatcher AMT-CR-PC-PRR