Tracks are organized into two groups of three, and two groups of two. Initially only the first group of three is available. Once you win each track on the medium difficulty level the second group of three is available. Win those on medium and the first group of two is available. Now win everything on hard and the final group of two opens up.
New bikes are also available with the second and fourth track groups.
Since the tracks generally get tougher as you go along, this means that for a lot of people not all tracks will be accessible. That's too bad, since some of the later ones are quite good. Most of the tracks are completely crazy, with turns and layouts you'd never see in real life, but they are mostly high speed affairs where your skills will certainly be tested. The game can be played over all of the usual network options. You can optionally include AI riders to keep things interesting if there are only a few players.