Basically it's a dungeon core becoming a train, appearing randomly over the world and causing mass chaos with random train goers.
The train dungeon would give them a status screen they can only use on the dungeon, and leaves an affect on their minds wanting to increase their power, making them if they get on or rebored want to stay longer and longer.
Basically the train would be the size of the infinity train, but with dungeons and the real world.