I do think that demonstrating the weathering techniques on a dark locomotive didn't come across very well in Jim Forbe's model photographs, or at least in their reproductions in the magazine.
For the truck sideframes he started with [1] the basic color (black or sliver), then added [2] a darker shade of black or silver and applied it to the inner truck surfaces (yeah, like I can get that kind of control from my single action brush!), then [3] a lighter shade to the parts furthest from the tracks (such as the steps). Finally he used [4] a wash of burnt umber oil paint.
The wash of burnt umber is a something that I've picked up that is pretty easy and gives good results on a lot of small details.