Someone once told me that speed was a determining factor but I tend to disagree as there are examples of both types in slow and high speed service. One interesting example is the N&W J class which I believe used a single for the first five (600-604) then two thereafter on what was arguably the most powerful 4-8-4 ever. My best guess would be that is has something to do with reduced friction/improved fuel economy but I honestly don’t know. Can anyone knowledgeable shed some light on this?
Thanks