Please step away from the instant weathering!
The car ends would be the dirtiest, as they almost never saw the car washer. Grundge is the key there, and washes and dry brushing will get you there. Pay attention to the diaphram faces as well, they had some pretty strange wear and rust patterns most of the time.
The underbody gets dirty/dusty as well. Another area that never really got washed. ATSF passenger cars tended to get a light tan dust color down there, that would also sneak up the sides during a run. Floquil Foundation is a good color to start with, and airbrushing it on the car side near the bottom, shooting upwards, will add a lot to the look of the corrugations. If it goes on too heavy, a wide brush with some Scalecoat II thinner on it will lighten it up as you drag it down the car side, cleaning off the tops and leaving it on the bottoms of the ribs.
Santa Fe did a pretty good job of hitting the trucks with silver paint to keep them clean. If you're modeling prior to 1968 or so, a light wash of black just to add shadow is all that is needed. Maybe a little more attention aroung the roller bearings.
Roofs often times weren't washed by the car washer. So atleast an even layer of dust is needed up there. A lot of passenger car weathering is subtle, because of the higher speeds. So even layers of dust, rather than rust spots and weather damage like freight cars.
Here's a later ATSF baggage car with some grundge.
