These powders are quite intense so please put some paper under the model you are weathering to catch any excess and reduce chances of making any mess.
Rust occurs when steel is exposed to the weather and a chemical reaction called oxidation takes place. The rust may occur as a generall overall discoloration on the surface that starts as a lighter more orange color. As rust itself is exposed to the weather it progressively becomes darker or browner in color. Therefore, several shades of rust on the same railcar are appropriate. Using a fairly stiff artist's brush or a cosmetic sponge(a small wedge shaped sponge, they come in packs of 30 for about $2.00) scrub some of the rust color onto the surface, working it into cracks and crevices. Rust will wash down the side of a railcar so put some near the top and draw down the side of the car with a bit a powder as if the rust washed down from the effects of rain and gravity. Randomly use a second color of rust to highlight other areas or put a bit right over some of the previously applied rust. Apply powder to roof walks, door guides, wheels, trucks, etc.
Dust and dirt usually start at the bottom of the railcar and work their way up from wind currents created from travelling down the track. Therefore the dust and earthen colors maybe applied near the bottom edge of the cars and sort of streaked upwards. Concentrate more color near the ends of the car and a bit of dust or dirt on the roof will help dirt up the car.
Experiment with the colors, blend different ones together to get new colors, be creatic. When you get a car to where you want it you may want to seal it with dullcote spray, this will permanently attach the powder and reduce chances of it rubbing off. If you do a car and do not seal it you can remove the powder with a damp cloth and start fresh.
The same applies to buildings. A bit of black on smokestacks, vents, etc. Blend a red color with a brown to get a dirty brick color to apply to sides of the buildings.
Experiment, such as a bit of powder mixed with a small amount of non-aerosol hairspray* to make a coolored paste. With a brush stab some paste onto steel grinders and when dry you get a flaky, almost 3-D scaly type of rust.
Have some fun and remember not all cars are rust buckets. Some cars maybe never rust and show only a minimum of dirt and others have much more. Do not forget the insides of gondolas, decks of flatcars, and the tops and sides of covered hoppers. Observe the railcars and buildings you come across and try to make some of your cars, buildings, engines, and equipment look more like them rather than fresh from the box. Your friends will be amazed with the improvements and ask "How did you do that?"
*Use the cheap superhold hairspray or a non-aerosol pump type hairspray. We owe this method to our friend Rich Divizio.