The rhythm of the train

Lately i have been working on manufacturing generic train cars that will run along the tracks. The intent is to have fours independent cars that will move in a rhythmic back and forth motion. Perhaps the Arduino will be used first, but eventually a series of switches within the tracks and the cars will redirect and change the rhythm. I created a quick mock up of the train chassis, and will be fabricating more detailed / designed frame soon. Here are some pics and drawings of what i have fabricated so far.