Locomotives are the focal point of every train. Because of this, I feel that they deserve some extra attention when it comes to finish. By adding a few basic weathering effects, the out-of-the-box locomotive quickly becomes an eye catcher on any layout.
A diesel locomotive is shiny when it first comes out of the paint shop, but time and use changes the overall appearance due to exhaust, fuel spills, weather, dust, and overall grime that is picked up running over the rails. When the diesel was first introduced by the railroads, it was pampered and kept as clean as possible. Some passenger diesels were actually waxed to keep that original shiny finish, and those with silver trucks were actually touched up before being sent out on the next run. Today's diesels are the victims of accounting. As long as they run, most railroads do not worry about their appearance. They are scheduled for repainting only when it is absolutely necessary, and in most cases, that can be quite a number of years. My best advice is to find photos of the locomotive and time period you are working on, and use this as a guide to your weathering process.
The first thing to consider on your diesel are the trucks. If they are shiny plastic, a coat of flat black will give a more realistic appearance. The wheels should be given a coat of rust. Brand new wheels should be bright orange, and older wheels will range from dark brown to grimy black. I use Floquil Roof Brown on my wheels, and have been very happy with the overall effect. Once the wheels are painted, clean the treads with an eraser, hobby knife, or Q-Tip dipped in solvent.
Couplers are generally some shade of rust. In the real world, the paint is always rubbed off them, and the effect of water makes them rusty. You can airbrush your couplers or dry brush them without interfering with their operation. Another product that works well is called Rust-All. This is actual rust particles suspended in a solvent that can be brushed on to produce an excellent rust effect. Be careful that you do not get paint into the working parts, or your coupler will not function properly. Rust can also be used on areas where the paint is worn off and the elements have attacked the metal such as on pilots, around battery boxes, on truck sideframes, etc.
Grills on our models look better when given a wash of black paint to bring out the details. (Remember a wash is usually 1 part paint to 6 parts thinner). Once dry, I over-spray the area lightly with some Floquil Grimy Black in my airbrush.
Exhaust soot is usually sprayed down the entire length of the roof of a locomotive. It is usually darkest around the exhaust stacks and fans. I also use Floquil Grimy Black for this effect in an airbrush. To add to your road-worn effect, some modelers use a black wash down the sides of their locomotives to represent soot and grime that has been washed down the side by rain. On black or really dark colored locomotives, a spray of Floquil Dust in an airbrush is really effective for bringing out details.
I spray the entire truck assembly and frame with a light coating of Floquil Earth to represent road dust. This is especially effective for bringing out the details of the flat black truck side frames. Don't forget to hit the pilots on both ends with this earth spray, as dust is always kicked up when the loco is in motion.
Locomotives that operate in areas with steep grades utilize dynamic braking. These create a tremendous amount of heat that can bake the paint off and around the grids. Airbrushing thinned orange paint vertically down from the grid will simulate primer showing through the paint.
Once you have completed your weathering, brush a streak of Poly-S Oily Black from the fuel filler down the side of the tank to simulate fuel spill. Diesel is messy, and every locomotive has this streak from when the hose is removed during refueling.
Cab units weather differently than hood units. On a hood unit, the dust and grime is fairly uniform along the trucks and frames. With a cab unit, the weathering begins in a heavy pattern behind the pilot, and then rises and feathers out along the side. The effect is almost in the form of a bow wave.
Whatever you choose to do, remember to study the prototype and make notes as to where weathering should be applied. Remember in the West your weathering will be dusty and light colored, whereas in the East and North, it will be darker and more grimy.