Clem, I've had great success with the bragdon powders. You can just build up the colors or weathering a little bit at a time. It gives one fewer surprises. The little makeup brushes one can buy at the drug store work well as applicators.
I don't use any fixative, but I don't handle the cars much. This K line Hudson was weathered with the powders
Laid off Sick gave you great advice on looking to prototype photos for your weathering plan.
If your modeling a period during the hey day of railroading the prototype trucks may not be too weathered. Diesels acquire a lot of yard or road dust, dirt and natural sun bleaching or fading.
