I have thought about this question on and off since my first post here, and have this further suggestion.
While details are what count in a really great model railroad layout, the difference between rust on a mainline is probably minimal, and varies enough to make any definitive rule impossible. I think what has been covered here is more than sufficient . . .
But there are differences that do matter and are there in the real world, the chief one I have seen by observation being that there is no vegetation in the active parts of a railyard - not a snip of grass has survived all the pedestrian and vehicle traffic between tracks, etc., whereas along a mainline you occasionally see grass and bushes that have encroached, and near a spur that is seldom used, you see grass - maybe tall grass even obscuring the rails.
Further, by observation back when I rode trains a lot, there is a lot of junk along the side of many mainlines - not muchwhen you are out in the country far from cities (i.e., halfway between Denver and Wichita), but particularly as you near cities there is the occasional broken tie just discarded near the track, a tire or two laying near the track, and litter and various crud like that scattered about, whereas in an active railyard, things are "workplace tidy" - kept cleaned up enough to work.
"If no one has ever done it that way, it might be fun to try."