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!
Phil S.