It seems like others have been getting the jump on me when it comes to recent modeling projects. I have a lot going on with Society (NKPHTS) business as well as other things that seem to keep my plate full. But others have some time and their efforts should not go unnoticed.
My friend Eric has shared some pictures of rolling stock that he & another modeler have completed recently. I can't emphasize enough how much more realistic pieces of rolling stock appear with a thoughtful coat of weathering. Eric's methods are rather cumbersome and quite detailed. There is no questioning his results but you need not use several different media to get satisfactory results.
Some streaks of thinned paint will go a long way to simulate the mud streaks that seem to be on all well-used rolling stock. Take most any mud-colored or earth tone paint and thin it with water... probably about 1 part paint to 4 or 5 parts of water. Using a medium tipped brush, simply streak this onto the car, working down as that is the way that water/mud would flow. Don't worry if the wash gathers in corners or along horizontal ridges, that's the way it would happen in the real world. Keep some paper towels and maybe a Q-tip or two handy to blot up the wash if you get too much on the car. Again, work vertically to simulate the effects of gravity on the roofs and sides of cars.
Don't forget to slather a little darker wash on those roof panels and wooden roof walks. Especially during the steam era, these surfaces were exposed to excessive soot and other smokey grime build-up. A little (or maybe a lot of) rust is also appropriate for older cars.
For detailed work, you might get appropriate colored artists' pencils. They are great for making small marks where you want to carefully control the effect. These artists' pencils are especially appropriate in white for ever present chalk marks that seem to be found on most all railroad cars.
As always, practice these techniques on older cars. You will be surprised how easy it is and how realistic the end results look. If you don't like the results, wash them off and try again until you develop a technique that you like and one that works for you.