For the train, as that is how a I'll be playing the game once I get a turn with it, perhaps what you could do is cover the mic with a finger when you aren't suppose to be playing, then lift up when you are. Could be tricky to get the hang of, but might work.
I sometimes even have problems with GTA:CW on the train - I'll keep calling cabs! I'll be stuck because it keeps stopping and saying to walk up to the chevrons. LOL. That game does have a microphone sensitivity adjustment, though. When I'm standing between cars, though, even the lowest setting isn't low enough!