What else did they have that would pull passenger consists of the required size in those years?
I'm sure someone here is a Southern aficionado and knows the progress of dieselization on those trains, which I don't, but that's the only thing that's a practical alternative. To my knowledge Southern never even considered larger or more modern passenger steam power than Mountains once there was a diesel alternative... even some of the fairly wonky types that Southern acquired in the '40s.