Tony Hawk's Underground - Platform: Gamecube

Perfect Rail Balance:
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Go to the options menu and in the cheat codes area enter letitslide.

Go to options, then go to cheat codes. 
Then you can activate it from the cheat menu.
keepitsteady - Perfect Manual 
letitslid    - perfect rails 
digivid      - unlock all videos 

Secret Characters:
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Iron Man     - Beat the game on beginner. 
Gene Simmons - Beat the game on normal. 
T.H.U.D.     - Beat the game on sick. 
Pedestrians  - Perform every gap. 

Complete 129 of 129 Goals to unlock a full cheats listing
Cool Specials  - Get 129 of 129 Goals 
Kid Mode       - Get 129 of 129 Goals 
Rollerskates   - Get 129 of 129 Goals 
Flame Grinds   - Get 129 of 129 Goals 
Always Special - Get 129 of 129 Goals 
Perfect Rail   - Get 129 of 129 Goals 
Perfect Skitch - Get 129 of 129 Goals 
Perfect Manual - Get 129 of 129 Goals 
Moon Gravity   - Get 129 of 129 Goals 

unlock hidden levels:
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Hotter than Hell - Complete Story Mode (on any difficulty level) 
Venice           - Go to Hawaii and go to the back left of the 
                   stage and ollie into the evil tiki statue 
Hangar           - Go to Moscow and turn right then go to the
                   right side of the building and jump in the 
                   5th window 
School 2         - Go to New Jersey to the train station and it 
                   will be on the right side at the very top.