Drew, while I am hardly the resident expert on anything, I like the general look of the car. As you've seen, Testor's enamal is not the best weathering paint since it's too glossy. There are two suggestions I'd make. First, do more of your weathering with chalks than paint. You can blend the colors better and get a more uniform look to the weather. If you don't like the result, you can use a damp cloth to wipe it off and start again, something you can't easily do with paint. If you do like the results, spray the car with some dullcote ad you're done.
Second, check out pictures of outside rib box cars and look at the ribs. The top surface of the ribs tend to stay clean while the panels between the ribs get dirty. You can fix this with a Q-Tip and some paint thinner on your model.