The threaded rod above the layout is what I have been using for several years with excellent results. A full construction article on how I did it appeared in the November 2009 issue of Classic Toy Trains.
You might be able to figure out how to make this fairly simple mechanism by taking a close look at the following picture. 12 volt DC motor from Radio Shack that is coupled to a 1/4" threaded rod using a flexible piece of plastic tubing.