All track comes from the manufacturer with the black coating common to steel that you would get at any steel supplier. When the rails are laid, if it is welded rail, they will grind off the ends of the rail to good penetration. If it isn't welded rail they just put it down, spike it and bolt it together. The railroad does not paint the rail when they install it and they never paint it after it is installed. The various shades of brown on old rail is rust. As the rail rusts it will change color, and the amount of rust will result in color variations. The tops of the rail on the mainline will be shiny because the traffic keeps the track polished and rust worn off. On a siding, the rails may be shiny on top if the siding is used a lot, they may be rusty if the siding isn't used much. I don't know of any way to model rust on top of rails without losing electrical continuity. I think the most noticeable difference between mainline track and sidings is ballast. The mainline will be ballasted. Passing sidings on the mainline will be ballasted, but for industrial sidings they usually just put the ties in the dirt and spike rail to them.