This is how I weather Atlas Code 100 track. It will also work on styrene, and polyurethane castings.
These are examples of weathered wood. These were taken at Roaring Camp Narrow Gauge Logging Railroad in Felton, California.
(Click on Images to Enlarge)
I use these materials. India Ink diluted in cheap rubbing alcohol. Make sure it is waterproof, different brands give different results. Flat white enamel to give a base for the India Ink stain. This demo I tried Artist Gesso and it works. Any flat white medium works, I used Kilz primer for the tunnel portal.
I add Waterproof India Ink to the rubbing alcohol until it produces a stain on a paper towel like this.
Virgin Code 100 Atlas track
Apply a thin layer of oil or WD-40 on the top of the track. The paint will be easier to remove after ballasting. Don't remove the paint until after ballasting, it will give a distinct indication where the track needs cleaning.
Apply a flat white base, spray on flat white enamel, or brush it on. This is Artist Gesso. Cover the rails also, painting the rails becomes easier.
Stain the rails with the diluted alcohol India Ink solution. Slowly build the color, start light and make it darker.
The flat white primer makes it easier to paint the rails with a brush. Flow the paint on covering the spikes and tie plates. I use Floquil Roof Brown.
This is a different brand of Ink, I believe it is Higgins it has more of a brown tint. There is also a sepia ink that I want to try.
Here is a close-up of the technique. It will work on styrene and other plastics roughed up to look like wood. I scrap the rails with a piece of 1/32 thick x 1/4 wide brass. The paint coating just pops off.
This is a polyurethane tunnel portal base coated with Kilz primer and stained with India Ink. It looked better but I didn't read the label and used India Ink that wasn't waterproof. It ran when I did the scenery and had to be redone in place. I think it is too dark. The arrow points to a PC board tie that has been treated with this technique.
Don't e-mail me about using grey paint. It will just look like grey paint. It doesn't give subtleties.