Matt H. just tipped me off to this fantastic truck design by Space2310. Probably about as small as you can make a powered truck. It's narrowgauge too! For two studs between the rails. But it is not just a truck,
Look at all of that detail in the vingette (and a novel ballest trick too). But that's not all of the innovations, look how the PF battery box is hidden,
Click on the pictures for the respective Flickr page. Space2310 has also included a video showing the train running around a loop.