Via the Scobleizer, we find the Invisible Train, a train that runs on wooden tracks (a la Thomas the Tank Engine, a known favourite in our household), but really doesn’t. Those crazy college kids:
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. 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."
I don’t think it would hold Gareth’s attention for very long — and Naomi would wreck the PDA with drool and repetitive chewing.