David Smith has built the smallest train I have ever seen. The Claim that it is the worlds smallest is probably true. Watch the video to see a good description of how the drive mechanism works. It is truly out of the box thinking.
"After carefully inserting the train-shaped tubing into its oval slot, I bonded the layout to a styrene support block that was in turn bonded to a mounting plate attached to the motor. This was an iterative process that involved fitting and sanding and fitting and sanding until the support block was exactly the right height. The mounting plate was made relatively large to provide a convenient means of attaching the mechanism to the layout under the building."