The next step can be airbrushing the underframe and trucks with a wash of dust or dirt and you're done.

Or add depth to the weathering by applying a pigment.  Most pigment has to be sealed to make it permanent - a top coating with Testor's Dullcote.  Some pigments have adhesive in them and do not need the Dullcote.

Airbrushing the Dullcote gives more control compared with a rattle can. Dullcote dilutes the effect of the pigment  so apply more than you think you need. 

I pre-paint the wheels, trucks side frames,  and sometimes the backside of the wheels and the exterior axle (depends on the car) with weathered black and brown rust (from Joe's Model Trains, Erie, PA - these are his track weathering products). The black first followed by the rust while the black is still wet - they merge into a nice color.

Apply gouache using the same process as the oils.  Use acrylic gouache thinned with Windex (it allows the solution to flow by breaking the surface tension of the water) and clean up is simpler.

Gouache can be your base coat - just spray the model first with Dullcote so the gouache has something to bite into. Use multiple layers and or colors of the gouache and chalks to build depth.  Roofs are often the first part of the car to exhibit rust.  Here, the end panels are already showing significant rust while the rest of the roof panels follow in varying degrees.

This auto carrier conveys a functional utility as reflected in the dirt and grime.  My model railroad is set in the southwest so I have more dust colors than northeastern dark grey grime.

All trucks receive a mix of weathered black and rail brown - side frames as a wash and wheels full strength.