In service the T&P boosters were used every trip west up 13 mile 1.35% curving Ranger Hill and even with the booster cut in speed never got to the booster's rating of max 15 mph where it HAD to be cut out. And, on 9 mile Baird Hill also 1.35% on curves they were used every trip. The heavy trains they pulled would not have made the grades without the boosters.
I remember watching 4449 in Ft Worth start from a dead stop and climb up a hill from the Rock Island yard to Tower 55. It was impressive both in sound and the train it was pulling up that grade. 4449 really was barking.