Yard tracks usually look darker, flatter, and less uniform than main-line track. These areas collect dirt, oil, coal dust, cinders, weeds, and debris from years of railroad activity.
Start with gray ballast, then blend in Yard Mix, black cinders, industrial dirt, and fine debris. Keep the ballast profile lower and less carefully shaped than the main line. Add small patches of weeds and darker material between the rails, near switches, and beside frequently used tracks.
Using several colors and textures together helps the yard look older, busier, and more realistic.