So far the process is Joe's Trains paint for the wheels, a mix of weathered black and brown rust applied as separate coats while the first is still wet. Same color carefully applied to to the couplers.
Many unloading guys just walk on the roof. Soon scratches begin showing evidences of rust along with spills and residue of whatever the commodity is. I weathered some cars lightly with this effect using a combination of Winton Oils' burnt sienna and burnt umber.
Contrast of heavy and lighter weathering. The dark areas are dabbed-on weathered black paint using a make up sponge. The streaking is from the Winton oils being applied heavily with a knife (a large paper clip works, too) and then dragging a turpentine dampened brush downward through the area. If you have too much or do not like the effect, rinse the brush, dip in the turpentine and remove the oil. Repeat as needed till you get the effect you want.
Here's the contrast on the roofs of the two cars. I still have to air brush the ends and apply weathering powders to soften and blend the effect. But the yellow car exhibits deep rust while the BN cars has normal rust from wear and tear. I extended the streak down the side as commonly seen.
Here is a minimally weathered car. Light rust around the roof hatch, several of the running board support brackets corroding at the weld, and 2 scratches down the side. The basic here is a gray wash to represent regular built up grime.