Yes, I've asked about doing so with tank tracks, but it's not just that. Looking at various railroad tracks in my locale, when I look at the sides of the track rails, I've seen blackish color with a thin coating of brown rust, definite medium brown rust, and red brown rust, which I take to be for how new the rails are. Should that apply to metal tracks in the sections where nothing rubs against them?

Iron oxide, commonly known as rust, starts of as an orange colour, almost yellow, and gradually becomes darker until it is a very dark brown, almost black in low light.
Railroad tracks are also exposed to oil/grease, old deposits of soot etc so they my have an additional "darkness" factor.

As for the rusty parts of tracks: it depends on the time that these parts have been allowed to rust.

Colour sample:

Full size image: https://upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Rust_on_iron.jpg/1280px-Rust_on_iron.jpg

I don't have any figures on approximate times for the different colours of rust to show up.
It depends on so many factors ....

When I lived in the desert as a kid we'd walk the tracks and the top of the rails were polished of course but the side were covered with iron oxide and damaged black paint. The was also powdered rust on the horizontal surfaces of the rail, plates, ties and the top of spikes. This rust powder was probably from bit of metal wearing off the train wheels and rails while the train was rolling along.