Stress on couplers

I know that the Rio Grande put a rear helper on westbound freight trains at Antonito because the combination of grade, tonnage and curvature would place undue stress on the couplers. I believe that 1,400 tons was the number used. Anyone with an old timetable could verify this.
The Chama-Cumbres segment also has a rating but I don't know what it is.
The third engine would be placed behind the tonnage rating of the lead pair. John Norwood explained this proceedure in one of his books This would prevent jackknifing. Placing the rear pusher ahead of the caboose would be the most logical place ~ unless there were several old, light, empty cars ahead of the engine.