When you choose and start a track from the title screen, you are racing on the "challenge race" version, an easy version of the track.

By placing at least 3rd in the challenge race, the next race is on the harder version of the track, called "Excitebike".

By placing at least 3rd in an Excitebike race, the next race is the next Excitebike race track.

For exemple : you choose track "1" at the title screen, so you'll race in Challenge Race 1, then Excitebike 1, then Excitebike 2, Excitebike 3, Excitebike 4 and finally Excitebike 5.

Other exemple : you choose track "4" at the title screen, you'll race in Challenge Race 4, then Excitebike 4 and finally Excitebike 5.

By placing at least 3rd in the Excitebike Track 5 race, the next race is Excitebike Track 5 again and again, until you lose.

I wanted to talk about that because it was very confusing for me the first days I played this game.