Bill, if you have an airbrush, pick up Testors Model Master in the following colors:
Or if you don't have an airbrush, pick up some artist's acrylic or oil tubes of those colors. Virtually any "weathering" color can be derived from these four. When I apply weathering from a tube (I use oils), I use a wide, soft watercolor chisel brush. It's very easy to blend with a dense soft brush like that, but it works best on flat surfaces. For complex areas like the underframe of a boxcar, I'll just shoot it with the airbrush.
I think the only thing that's not working with your weathering is that you're treating the wheels and couplers as different colored surfaces than the trucks, draft gear and underframe because you "know" they are different types of surfaces. That is, the couplers and wheels are left unpainted so they'll rust and the other surfaces simply collect dirt. It makes perfect sense that they should be painted differently, but it works best to paint them all one uniform grimy color first then add the rusty effect to the wheels and couplers later by drybrushing. Of course, this is probably best attempted on a car that's supposed to be several years old on your layout and would have collected layer after layer of road grime. The rules are different for a newer car.
Anyway, I think you've done a fine job of weathering. If you can overspray the underframe assembly from front to back with a "unifying" color, such as a burnt umber based road grime color, I think you'll be less distracted by that underframe and more impressed with what's on top of it.