Figure1.1 pictures a railroad track on which are found a locomotive and two railroad cars.
Car B at the right has just been filled with coal from the conveyor belt; car A at the left is empty. The tunnel is large enough to accommodate either car but not the locomotive, L. Furthermore, each car is longer than the tunnel so that, when in the tunnel, it is accessible to the locomotive from either side.
Using the locomotive to push or pull the cars, how can the two cars be made to switch places and the locomotive end up between then?