The Invisible Train

The Invisible Train is a mobile, collaborative multi-user Augmented Reality (AR) game, in which players control virtual trains on a real wooden miniature railroad track.

via USC Interactive Media Division.

These virtual trains are only visible to players through their PDA's video see-through display as they don't exist in the physical world. This type of user interface is commonly called the "magic lens metaphor.

Players can interact with the game environment by operating track switches and adjusting the speed of their virtual trains. The current state of the game is synchronized between all participants via wireless networking. The common goal of the game is to prevent the virtual trains from colliding.