It's not uncommon for developers to leave things behind during the course of a game's development. As we've seen with Bloodborne and Dark Souls, sometimes that comes in the form of cut enemies, discarded items, or new pathways. In the case of Mario Kart Wii, it's a beta version of an entire track.
If you go digging in a game's files, you'll find all sorts of wonderful things. YouTuber KevinVG207, along with Atlas and Riidefi, discovered a Course.0 file years ago, contained within a koopa_course file.
Using a text editor, the course.0 file has some fun details: namely that Yabuki Kosuke created the file, the producer of ARMS and a long-running Mario Kart developer.
KevinVG207 kept chasing it up, and discovered — by digging through a ghost track file uploaded by an expert ghost file uploaded by Nintendo — that the JMap file was created six months before the game was actually released. But even better was this little nugget: data for collision materials.
Image: KevinVG207 (YouTube)
The information there was enough to start working on rebuilding a playable version of the beta Bowser's Castle track. The general layout is roughly the same, and the JMap file even contained enough data about item placement to rebuild that as well.
It's a pretty early version, with some collision problems. But some of the item boxes are in different spots, and there's a couple of small tweaks to ramps and roads.
Generally, the early design of Bowser's Castle was retained for the final version of the game. But as always, it's interesting to pick apart and see the data that developers left behind. It's also fascinating seeing the amount of dedication people put in to get leftover data working again, with AI bots as well.