Have a go using Gouache (a water mixable color available from art stores) for adding a pitting or streaking appearance to freight cars.  Use a very fine artist brush to put a small dot of Gouache on the spot where you want to add a rust pit that has streaked down the side of the freight car. Let the Gouache start to dry. Then dip a wider brush in water (or Micro Sol Decal Set) and then pull the brush gently downwards from the dot. This will create a streak.

You can also use Gouache to create much thin washes of rust or dirt. Try using white gouache to give a bleeding white effect on wagons.

Another idea is to prepare a thin Gouache wash. Apply the color to the car so it is is wet. Then sprinkle a little salt (kitchen table salt) on the surface. Wait for it to dry, then rub the salt away with your fingers. Then touch up the car with dabs of gouache to finish off the pitting. Always test new techniques on an old car first. You wouldn’t want to make an mistakes with one of your best cars.

More scenery ideas in my new ebook available now from http://www.model-train-layouts.net