The track is (purposely) not flat and not smooth. It will be slightly banked to encourage passing opportunities. That way, it's more exciting, since the marbles aren't actually "driving". There is no interference.
Giacomo
Just watched one race (Savage Speedway GP S1R1) now, and I must say, them marble-slipstreams are very powerful!
Do the marbles get any control from the players at all? I saw on the front stretch one of the marbles suddenly sharply change direction!
3:50 in, 3rd place marble changes direction suddenly one the straight!
But even if it wasn't, that team is called the Savage Speeders. Been watch M1 for a while now. Savage Speeders are like the Patriots or Astros of Marbula One. Always bending the rules/cheating. Just kidding, of course, but they seem to get lucky bounces sometimes.