Everytime I do trainclip I'm stuck, not even able to move, on the train with G-Man. Other times I get stuck directly in the cart behind. Or there is a white screen. What am I doing wrong?

make sure you start holding the minimize button after g-man starts moving backwards and you hold it until after you hear the train horn

Okay, I found it out since that time and it's not holding it down like, long long after the horn.