Elefant

Thats quite simple indeed. I used Tamiya weathering kit A and B. You can use regular pastel but I liked the colors chosen by Tamiya.

First of all, I washed carefully the metal tracks, after a full dry I applied a metal primer (white in my case but I'd recommend the gray one).

I mixed a gray, a brown and a tan to get a dirt color (I use tamiya acrylic paints with my airbrush system). I painted the tracks with that dirt mix.

Now the tamiya pastels....I mixed the rust pastel with a little amount of tamiya acrylic thinner to get a very rough mix that I applied on my tracks with a regular brush. I used a cotton swab to remove the paint from the contact points of the tracks so I get a natural metallic finish.

After that I used the sand pastel (poured as a powder on the track to get that dried mud effect)

Hope this helps.