The Mystery Train is a vehicle of sorts found in Mario Party 4. It is featured in an event on the Boo's Haunted Bash board. The Mystery Train is composed of four ghostly coffins that shimmer in a translucent blue and hover above the ground. It can be found in a cemetry-like environment in the northwestern corner of the board and is managed by a Boo. The event will be triggered automatically once a player passes by.
The Mystery Train event is affected by the presence of the Red Boos on the board. The player will get a chance to ride the train if the Red Boos are absent while the event is triggered. The player will then get on one of the four coffins and hover along the field. Any opponent that the train meets during its way will be picked up and follow the train as well. The person in the lead can decide in which direction the vehicle will move until it reaches a graveyard where the players will be released again. The player who started the event will continue to move forward, depending on the amount of steps he or she has left.
If the event is triggered while the Red Boos are present on the board, the Mystery Train will not depart. The Boo in charge will appear and dismiss the player, claiming that the Red Boos are spoilsports and that the train will not move until they have vanished again.