Hey Chris. Weathering is a lot of fun. I'm a believer in chalk (artist pastels) and genuine Testors Dull Cote. While the results won't be up to the level of some weathering specialists who use advanced technique, the models will look great and a fleet of weathered units is within timely reach.
If you have a Hobby Lobby nearby, you should be able to get the pastels in a kit with six 1/4" square x 3" long sticks for under $10.
Here's a repost of my weathering method. Be sure to remove any windows (or tape them) to keep the Dull Cote off unless you want a dirty trailing unit in a loco consist.
Keep in mind, I'm no expert, and there's usually a right way, a wrong way, and Grande Man's way.
The "victim" and supplies.
The first step with lighter colored models is to "soot" them with black chalk. Cover the whole model. I prefer an angled brush to work chalk into all the nooks and cranies.
Using a paper towel dampened with "wet water" (small amount of detergent, used in scenery work), clean excess soot using a verticle wiping motion. The idea is to leave streaks where they would accumulate on the real thing such as around structure that's above flush.
At this point, coat the model with Dull Cote. Since the wheels have been removed, try to keep the spray out of the truck journals. Dull Cote "sets" the black previously added, while at the same time, toning it down somewhat. It also creates a surface that "grabs" lighter colored chalk soon to be added.
Using the same brush, add dirt and rust. Since we model the West, little rust is used. Lighter grays can also be added to the lower areas where ballast dust would accumulate on the prototype.
We use Floquil Rail Brown to paint wheels. Be sure to keep paint off the bearing area and treads. The trucks are dry brushed with Rail Brown and chalk highlights are added.
Presto! The finished hopper at CF&I's Split Rock Mine #4.
