Here's a "before" picture of the three cars:
See how pristine and shiny looking they are? How many real world freight cars look like that? Not very many. I start of weathering by brushing on a black wash made by adding a few drops of India Ink to a bottle of 70% rubbing alcohol. Once that dries I spray on a coat of Testors Dulcote.
After the Dulcote has dried thoroughly, I take some pastel chalks, grind them up and brush on the dust. I start off with a coat of white to fade the paint a bit. I follow that up with some brown for rust, a mixture of brown, black, white and yellow for dirt and black on the roof to simulate soot from steam engines.
After you apply chalk weathering you have to seal it with a second coat of Dulcote. Usually this makes about two thirds of the chalk vanish.
Chalks Before Dulcote:
Chalks After Dulcote:
Here's an "after" photo of the three freight cars:
And a better view of their rooves: