From the Super Mario Wiki
Rainbow Downhill is a course in Mario Kart Arcade GP. It has a similar theme to that of Rainbow Road, from the other Mario Kart series. Rainbow Downhill has the same starting point as Rainbow Coaster, but a different area. This area has many hair pin curves and bumps. Mario Kart Arcade GP 2 has the same tracks as its predecessor, but no coins at the Jumping Shortcut.
Rainbow Downhill and Rainbow Coaster are the Arcade GP series counterparts of Rainbow Road.