The Fawful Express is a train found in the game Mario & Luigi: Bowser's Inside Story. In the game, the Fawful Express gets ready to leave Bowser's Castle while Bowser is standing on the tracks, looking for the safe with the second Star Cure. As Bowser tries to stop the train from crushing him, he trips, and is crushed into a rock. There is then a giant battle (3rd giant battle overall) against the Monty Bros. and the Fawful Express.
In battle, the Fawful Express doesn't actually fight by itself but mainly tries to get away from Bowser at all costs. Sometimes, it will hide in giant trees where it starts to get offensive. It can also heal itself at one point. If Bowser doesn't derail the Fawful Express in time, he will fall off a bridge and lose the battle.
Due to the Fawful Express' height, Bowser's punches cannot touch it so he must use only his fire breath to damage it. Bowser must use his fire breath effectively to reach the highest damage. It is also wise not to take so much damage during the fight as wasting a turn to heal may cost Bowser the battle.