Ok.
Start with washing the tracks in warm water and dish detergent with an old toothbrush. I use a bath of isopropyl alcohol and agitate the parts in that. Denatured alcohol for styrene and resin parts.
Enamel spray paint the tracks with flat black. After this base coat cures, follow with washes of acrylic dark browns and rusty browns.
Install the tracks.
Then wash in earth tone pigments of your choice.