More and more steel cars in the Southern Railway stock around World War I meant that the existing Pacifics were not powerful enough for all lines. This was especially true for lines like that between Washington, D.C. and Atlanta, Georgia that was relatively hilly. This is why the Southern ordered class Ts Mountains from Baldwin.
In 1917, 23 were directly delivered to the Southern. Seven more went to its subsidiaries, to be precisely five to the Cincinnati, New Orleans and Texas Pacific and two to the Alabama Great Southern. Like the Ps-4, they got an apple green and gold livery in the twenties. Soon they were complemented by 33 USRA Light Mountains, which were called class Ts-1. After their retirement in the early fifties, all Ts and Ts-1 were scrapped.