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Rust is just another name for steel oxidation. The color can range from a very light brown to the characteristic deep brownish-red rust seen on the running rails. The polishing action of the wheels on the running rails gave a better surface for oxidation to attack. The guard rails will take much longer to get that red rust look but they will if left alone long enough.
The guard rails will always be about the color seen in the photo unless they are very old or brand new. The running rails should look that rusty for abandoned track but not for track in use. Track in use usually has rust colored sides and a a shiny top surface from being polished by the steel wheels. Looking at the condition of the ties and the size and amount of the overgrowth on that bridge, I'd guess it's been at least 20 years since those rails have seen a train.