Between mainlines in crossovers. Between mainlines and sidings. 32-inch minimum radius. Pactra Dark Earth is a good base color. Your choice, but usually medium gray.
Sidings are any tracks except the two mainlines.
Its purpose is to facilitate the movement of trains into your sidings. It may consist of one track or all of your sidings. But those that do, can link during operating sessions and shows, to form a third line for trains to operate on. Siding track planning is your option. Tracks may rise above or drop below mainlines.