After watching some of Joe Fugate's great videos, I really liked his weathering powders (dried tempera paint mixed with plaster of paris). I thought I'd give it a go on rolling stock, as it seemed like it might be a good way to hit a lot of cars quickly and easily. What makes this great is once brushed on, simply wet the powder, and the plaster makes it adhere to the model.
I started by making three colors from his video - light gray, light tan, and finally sooty black. I adjusted the recipe for sooty black a bit, to get a darker color.
The car before any work.
One coat of light gray
another coat of light gray
one coat of light tan on the bottom 1/2 of the car, sooty black on top
And the final with another heavy coat of sooty black on top
I'm not sure if it's the camera, but it looks better in person, when you'r not zoomed in, like this picture here.
Joe suggested running a wet, stiff brush in a vertical fashion to even out the weathering, here are my results from that. It definitely evened things out.