With the transition from "tin-plate" to "scale" in the 1930's ..modelers we're looking for reasonably priced Tinplate engines that they could modify for scale operations ..the American Flyer Hudson, 060 switcher , and Atlantic/Pacific ..from 1937-1940 fit the bill for many .
Scaled at roughly 17/64ths , priced under $15.00, they provided a good basic engine from which many modelers and hobby shops would turn down flanges , regear, re-motor , insulate for 2 rail or add sweeps for outside 3rd rail use.
To see a video of an Atlantic / Pacific modified into a ten wheeler please see link
https://youtu.be/EqJjbIL0T6I
Cheers Carey