Hi Rob - I found that the hardest thing a beginner has with weathering a model is screwing up the courage to attempt it! And, I'm including myself here from experience!
I suggest you take one of your coal wagons apart and throughly clean the body and chassis with warm water and dish soap, rinse and let dry. Then give it a go. First, using a brush, paint any hardware on the body like hinges, iron straps and stanchions an appropriate color such as grimy black or boxcar red. You will be amazed at what a difference this alone will make. Then use a fine brush and "dust", "grime", "rust" or other RR color paints that you highly dilute with thinner to add some pigment into crevices, journal boxes, etc. Go lightly at first. You can always add more. I think you will be pleasantly surprised with your results. Finally, give it a light coat of Testors Dullcoat or equivalent. Remember - if you don't like it you can always do it again!
Yeah, highly dilute your paint and don't over do it at first. I see 'weathering' jobs done with brush stippled black paint etc. and they don't look good at all. I have also seen airbrushed paint that has not been diluted and it looks all finely spattered instead of fine dust. Another is black locomotives with white and rust airbrushed in very obvious spots all over. It is an art form in itself but you can't hurt a coal car too much so give it a go. You can put a base of rust on the metal parts then use various techniques to add some flaky paint on top. Depends if you just want dusty working cars or ones with rusted distress. Use reference images of weathered prototypes from Google as a guide. A light dusting effect from bottom up seems to look good. Wooden hoppers will have bare wood exposed on edges etc. For metal areas look into the salt technique. It can be done in various ways.