A train is designed to travel in one direction only, forward. Its path is forever dictated by the track beneath it. It has no choice. Thus, collision with other trains is avoided. The car on the other hand, is designed to maneuver in infinite directions. This is not a problem until other cars are added to the equation. They, of course, are also moving in infinite possible directions. Indeed, on one road at any one time there may be thousands of such creatures, each moving on their ...