...when it comes to weathering, there is no 'correct' way to do it, once you've decided to take the plunge into weathering a locomotive, piece of rolling stock or structure, the best thing to do is take a look at the real thing and try to duplicate what you see. I've tried using an airbrush to apply layers of 'dust', I've also accomplished the same task using hand brushed chalks or pigments and applying a coat of dullcoat as a sealer. I drybrush rust stains and use chalks and pigments to give everything an appropriate coat of 'grunge'...keep in mind that everything is clean when new, and things do occasionally get washed, so a good rule of thumb is to make sure your fleet isn't too uniform in appearance - try to have a mix of clean, lightly and moderately weathered, quite dirty, and a few that look absolutely grungy and totally forsaken...take a look at your average mixed freight the next time your out railfanning and that is usually what you'll see prototypically.
Lorne Miller