I agree with Fred, burnt umber is a great, cheap, water wash color to start with. You can add lighter rust streaks here and there or grey dust with an air brush or dry brush. Auto primer is about the same, maybe a bit lighter, but I prefer the water based solution. I've tried brushing, airbrushing and spray paint and found it about as fast to squeeze out a big blob of burnt umber and saturate the ends of the bristols on a cheap 3" brush, then jab at the track with the bristol tips, then wipe off the top of the rails.