The number of axles per truck is usually a pretty good guide for rolling stock and diesel locomotives. Diesels with four-wheel trucks are all that I run on my layout due to the tight-radius curves, and I also tend to stick with smaller steam locomotives such as 0-4-0s, 0-6-0s, 0-8-0s, 4-6-2s, and the like. It's no problem for me because I prefer smaller motive power, all the way down to various type of "critters."
Most all of my rolling stock is also of the four-wheel-per-truck variety.
