There are many ways to make your models look old and well-used, but none is more reversible than chalk. If you've never tried weathering before, this total reversibility should give you the confidence to start.
Weathering chalks are available in a variety of colors from model companies or at art supply and craft stores. You can buy chalk as sticks or powder. Sticks are usually cheaper and you can turn them into powder easily with a small piece of light sand paper. Earth, rust and grey tones all work well for simulating the dust, rust, grime and grit of railroading. Study pictures of the prototype for inspiration.
Clean the car with a soft cloth to remove any fingerprints before you start, or your model will end up looking like evidence in a detective drama. You can also wash models is a light wash of detergent to remove heavier coats of dust and oils. Cheap plastic gloves are handy for keeping the oils in your hands off the car while you work.
Chalks generally adhere best to a flat finished surface. If your model has a high gloss, use a flat finish spray to lessen the shine first.
There are several weathering techniques that will yield different results. Combine these techniques to produce endless combinations of effects and mix it up to make each car unique.