I believe that with this site's collection of Mario Kart classics, it is baffling that we are missing not just Super Circuit's Rainbow Road, but also Mario Kart 8's and Mario Kart 7's. In fact, there is not a single Mario Kart 7 song to my knowledge on here!
You can find an arrangement of SC's Rainbow Road on my arrangement thread here (https://www.ninsheetmusic.org/forum/index.php?topic=8570.0), and Sebastian has a MK7 and MK8 RR sheet here (https://www.ninsheetmusic.org/forum/index.php?topic=4911.0).
Neither of us have submitted these sheets yet (a somewhat lengthy process), so they aren't on the main site.
In my Arrangments thread (linked in my signature below) there should be numerous Mario Kart 7 Arrangements, as well as MK8 and GBA Rainbow Road. Some of them are probably sub standard though.