Train Exhibit

The Train Exhibit is a room in Treacherous Mansion in Luigi's Mansion: Dark Moon. Located to the north east of the mansion, it's accessible via the Study to the south. Inside the room, there's a display near the centre where the Big Boo boss battle takes place, a train track around the outside with a rideable train and some decorations to the sides that can be interacted with via the Strobulb.

History
In E-3: A Train to Catch, reaching this room is the entire aim of the mission. It has a couple of ghosts Luigi will need to defeat to light up the area, and then looking into the display itself will end the mission. You can also get the mansion's 13th gem here. After clearing out the ghosts shine your Dark Light on the missing piece of track in front of the display case. Then get on the train to go around back and collect your gem.

In E-4: Ambush Maneuver, the battle with the Big Boo takes place in the display located in this room. Because Luigi has been miniaturised and teleported into the display via the Pixelator, there is no way to exit the display/boss arena or explore the rest of this exhibit in this mission.

After these two missions, the Train Exhibit is irrelevant for the rest of the game.