how does that work?
you will need several small containers of water,a couple of small brushes, chalk in multiple colors(particularly charcoal-black, and reddish brown), q-tips, and artists fixative spray. All pretty cheap.
You apply the chalk directly into the corner area of your rolling stock and locos, even buildings, occasional draw a line into the main area but only if it is reddish brown, charcoal, or a color close to, but not exactly the same as the original surface. Now moisten a brush, slightly and father the areas. the ares it will settle in are lower recessed areas, it might streak. Well what's going on is the same way how grime and rust settles and streaks on an object that gets rained on as often as railroad cars do, you are using water and gravity to simulate water and gravity! If you think you've overdone it a little in one area, use the q-tip as your eraser. Not happy with the whole thing, just a bath of warm sudsy water will remove it. When you are satisfied, use the artist fixative 12 -15 inches away from it.
If you want clean rolling stock again, just give it a sponge bath in warm sudsy water two or three times.