Diesel, it's just cork---it elevates your track above ground-level and also absorbs vibration, making your trains run more quietly. Depending upon what you want to spend, it can be formed with angled sides to resemble the roadbed built up under real RR tracks, or it can be just sheet-cork you cut and modify as you see fit. Usually, people choose between just attaching the track directly to your ground surface (foam sheet, painted wood, etc.) or creating a more realistic, raised surface. Older stuff like my AF had molded rubber. Use what you see fit. if you're going the cork route, you might look at rolls of cork plumbers use to wrap pipe.