Articulated steam engines have been in existence for a very long time, since the 1890s. But during WWII, the US was always in dire need of heavy locomotive power, as all the major freight haulers resorted to bigger and bigger steam engines, culminating in Southern Pacific's 4-8-8-2 cab forwards, Norfolk and Western's Y6 2-8-8-2s and the Union Pacific 4-8-8-4 Big Boys.
Now, what if at this point, some intelligent soul had seen the Garratt design, particuarly the huge Garratts of East Africa and Australia, and wondered if those would work in America? They were known for massive pulling power......