Many gamers consider Nintendo's F-Zero X to be the best non-kart racing game on the Nintendo 64. Japanese players were treated to a double-dose of Captain Falcon thanks to an expansion pack released for the 64DD add-on system that added two new GP cups and a track editor that allowed players to design their own custom F-Zero X race tracks and even edit the existing courses. The new cups, DD-1 and DD-2, were never released outside of Japan and have largely been unseen by the rest of the world. At least, until now.
A dedicated independent programmer going by the name of BGNG who dabbles in the Nintendo 64 emulation community has recovered the 64DD cups and has made them available to fans to play via Nintendo 64 emulators. BGNG has created a Windows program that contains the DD-1 and DD-2 cups from the F-Zero X Expansion Kit. This program can patch the F-Zero X game data to rewrite the game to include the lost tracks in place of the existing tracks. For example, the program can overwrite the courses of the Jack Cup with the courses of the DD-1 Cup.
I love to see fans of classic games dig deep and pull up game data that was thought lost or inaccessible (Hot Coffee issues notwithstanding). I hope that if Nintendo offers F-Zero X on the Nintendo Revolution's classic game download service the company will add these lost tracks into the game. Now since it is illegal in the United States to link to a website offering unlicensed copyrighted data (in this case, the lost tracks) I won't be able to direct you to this program. It is out there though if you want to search around and give it a try, so please do not ask me where you can find the patching program, the F-Zero X game data, or a Nintendo 64 emulator.