At the main menu, use this code to unlock Speedway Reversed bonus track:
hold L and press X, Square, Triangle, X, Triangle, Square
Unlockables:
Race as Bosses: Defeat DJ, Snot Rod, Wingo, Boost, or Chick in their Boss race in Story mode
Race as Doc Hudson: Win the Gran Prix Cup 1
Race as Sheriff: Win the Gran Prix Cup 2