Over time I think I have improved my skills. That came with a lot of trial and error runs; just testing to see what looked right. I use Paint.net as well, and there are plenty of useful tools you can use that can aid in your weathering techniques.
For me, I mainly use photo-real decals. You can tweak the color of the decals and also edit the transparency of the layer that they are in so that the decals will blend in well with your texture. Sometimes, I will use the paint brush and edit the transparency and hardness to a desired setting. At a low transparency, black works well for adding grime to the texture, and white can be good for making your texture look more pale (as if the paint was fading).
There are many ways to weather locomotives, and many folks have their own techniques. I enjoy weathering because I end up doing something a little different each time, learning as I go along.
I have some photo-real textures that I can send over to you if you want them. I thought I could also include some example .png files that I used recently so you can look at what I have been doing as far as layer transparency, decal usage, etc. Just p/m me if you are interested.
