From the Super Mario Wiki
The Bad-Breath Express is a locomotive operated by Bowser that appears in the game Mario Party 8. It appears exclusively on the Shy Guy's Perplex Express board, and bears resemblance to the Koopa King from Mario Kart: Double Dash!!, similar to how the Barrel Train resembles the kart of the same name.
The Bad-Breath Express appears whenever a player lands on a Bowser Space, replacing the train's current locomotive, which could be either Shy Guy's standard one, or Donkey Kong's Barrel Train. The first player who reaches Bowser's locomotive will be forced to give a Star to him. However, if the player does not have any Stars, Bowser will reward them with ten Coins.