Gidday Dave. It was not my intention to try to "Teach my Grandmother how to suck eggs", far from it.[:D]
What I immediately thought of when I saw your original comment was of the" New Zealand Government Railways" and their purchasing policies after 1900. Including the “Q” s the first class of 4-6-2 Pacific’s in the world , introduced 1901 and the “X” 4-8-2 s arguably the first “Mountain” class to go into service anywhere, introduced in 1908, out of the 24 classes totaling 486 locomotive introduced until the demise of steam in 1972, only 4 classes, comprising of 96 locomotives did not have 4 wheel trucks. ( 13 were 0-6-2T for branch line general service; 41 were 2-6-4T for suburban passenger; 18 were 2-6-4T for main line freight and the final 24 were 2-6-2 "Shunting" (Switching) Locos.
Now as that thought entered my head, I also wondered about the relevancy of a small obscure narrow gauge railway on the other side of the world to your observation, and ( here’s a sweeping statement for you) while no self respecting modeler that I personally know modelling the US “Transition Period” doesn’t have at least one “Mike” or “Decapod”, (personally I wish I could justify having one of those “handsome” Nickel Plate Berkshires), I did also think about the reasons in the development of the “Mountains” and “Northerns” in the US