*Resurrection of an old post.*
Regarding weathered trucks:
That works, but there are great variations in color on trucks. I start with a grimy black base, or flat black from a spray can. Next, I hit it with various "pastes" of AIM powder & various liquids, such as hairspray, water, and rubbing alcohol. (Tap water doesn't work so well, but distilled water does.) Dullcote follows.
Be generous with applications of rust.
For wheelsets, there are greater variations of rust and grime. There are often splashes of mud and other natural substances. I use Polly Scale grimy black, followed by a generous application of rust powders. I use hairspray or alcohol to move them around, dipping the tip of the brush slightly in the container.
For more on trucks, go to
http://www.modeltrainsweathered.com/forum/viewtopic.php?t=817
Matt