Clem it really depends on the look you are going for. Either way, they must have a flat finish to grab the chalk/powder or some type of textured surface. Dull-Cote, or just paint them flat black.
Obviously these are freight car trucks, but they were all painted flat black or brown to start. Most of it is powder after that, or a combo of oil paint stippled on, followed by powder, after the oil paint is completely dry.
I don't think you want your locomotive trucks to have that much texture but you get the idea. 
It always helps to have a photo of what your are trying to duplicate. So find a photo of an RS11 similar to what your are going for. Then just try to match the colors and patterns.
