Model railroading has grown, over the last thirty years, from an around the-tree toy to a full-fledged creative hobby. Many of the least expensive plastic models have as much detail as hand-crafted museum models. In fact, the majority of the model railroad kits and ready-to-run cars and locomotives are as detailed as any modeler would want. Somehow, however, none of this equipment looks quite the same way it does in real life. Part of the reason, of course, is that it takes some skill and experience to achieve a realistic scale model setting for these models, but there's more to it than that. Every model you see on your dealer's shelves is brand new - you expect it to be brand new - yet, only the smallest fraction of any real railroad equipment is "brand new." One trip cross country through a haze of diesel smoke, wind-blown dirt, and a quick wash of night time dew or rain to "set" the layer of grime onto the sides of the car or locomotive and that "new", equipment looks very much used. Given a few months or years of exposure to the elements and to the drips and overfill slop of loading, the railroad cars and the real railroad's equipment looks darn near derelict.
No modeler is anxious to make his "brand new" car or locomotive a gritty and grimy mess - it's hard enough just to keep the dust from collecting on the tops of the equipment. We don't propose that you dip your new models in a vat of grease or a box of dust to get a truly "authentic" layer of real railroad grime. We would suggest that you look at your model railroad through the eyes of an artist. You are, after all, trying to create a three-dimensional portrait - all be it an action portrait - of reality. The one talent that makes an artist like Norman Rockwell's work so life-like is attention to what you really see when you look out at the world. Try looking at your railroad models with a thought toward what the real thing looks like. Only about one car or locomotive in a hundred looks as clean and shiny as those fresh-from-the-box models. The kit makers don't supply the "grime" when they paint the sides of the cars or locomotives so its up to you to add it. "Weathering" a piece of model railroad equipment is very much an "art." The trick is to add just enough thinned paint to give the appearance of rain-set dust without completely obliterating the lettering on the side of the car or locomotive. We'd recommend purchasing an artist's tool for the job; an air brush. Your local hobby shop or artist's supply stores should have several brands in stock. The air brush is nothing more than a miniature spray gun of the type used to repaint automobiles. You'll need some source of compressed air for the air brush. Sears, Roebuck and other such chain stores sell small air compressors for under $50, or you can start with the cans of compressed "propellant" (aerosol-type cans without any paint inside) sold by Badger through hobby dealers. The air brush (Thayer Chandler and Badger are most often used by modelers) will cost another $25 or so. Its a relatively large investment but one that can improve the quality of all of your modeling work. The better air brushes have adjusting valves that control the "fan" or spread of paint down to the size of a pencil point. It's this control of the spread of the paint spray, plus the advantage of being able to mix the paint you use in any color and with any amount of thinner, that makes the air brush such a worthwhile tool.
The dirt and grime that appears on any real railroad's equipment is a direct result of how and where the equipment is used. Diesels, for example, quickly accumulate a coating of black diesel exhaust smoke. All of the equipment will accumulate a layer of dust and mud kicked up from the surrounding ground as the train thunders by. The freight cars often have an external indication of what they most often carry inside. Many box cars, for example, have a light grey haze over their sides and roofs from being parked alongside a cement plant while loading sacks of concrete. Tank cars have drips and runs of black oil or light green chemicals running from their domes and down their sides. Your best' "instructor" is the real railroads; take some time and, maybe, some color photos of the more "typical" cars and locomotives to see the colors and patterns that the dirt takes, The ends of the cars are usually a bit dirtier than the sides since the wheels kick up a spray that collects in greater quantities on the car ends then on the sides. A majority of the cars will have a greater accumulation of grime along their bottom edges than along the tops for the same reason. Try to find as many different patterns and colors of grime as you can to apply to your models - the one outstanding similarity between the different types of real railroad grime is that it varies so much in location, color, and thickness from car to car.