Thanks!
Bob2 and Mark, the weathering was carried out with acrylic paints and very few chalks.
1) a brown overspray with highly thinned acrylic paint, I use Revell but the brand does not matter. This is the only moment when an airbrush is used. All Components are disassembled so you can roll the mechanism in your booth and for better handling. It is important to apply more of the brown on the lower portion of the model, becoming lighter towards the roof. Oh look how crude the wheels are! 
We will have to change that later 
2) The color is then modulated with a flat brush and isopropanol alcohol, at that time I used methylated alcohol. they both re-activate the color and you can brush most of it away, creating streaks. I try to leave more over the rivets and generally on the lower portion of the model. Attention here - the alcohol might eat up the decals, they have to be sealed carefully. The sealant should resist the alcohol, of course - I will make tests before I weather an other. All the factory painted plastic models like Atlas and Intermountain cars have no problem with alcohol.
3) Seal your brown layer with a clear coat. Krylon Matte Finish or anything that protects against the next coat.
4) The most important step is to apply a THIN wash of a light grey with your brush. I use pure thinner to wet the model before applying to avoid drying edges, they can ruin the job. All of this can be modulated with alcohol but I find it easier to apply it nicely in the first place. This step is subject to really developing your own technique and to use your HO corpses for test runs.
It's all about removing paint and shoving it around. Once you're happy the components go back together and some pigments are added. At that time I had no concept of soot and grime on a steam engine, today I would add more black pigments around the stack and on the roof.
All the side rods received some burnishing on the outside. This locomotive ran through Nebraska at 60 mpH and is now going to be serviced at Cheyenne, Wy. The shine of the rods would have looked still too fresh with just the above weathering.
I wished I had converted the engine to p:48 before all the weathering. That way around would have been much easier.
