A small glass ball loops around from the top to the bottom. the loops have been made using LEGO rail-beams. Then it falls into a wheel that brings it back to the top. The working can be viewed in a small Mpeg-Movie (1 Mbyte).
This view shows details of the wheel and the mechanism for pulling the weight without stopping the machine. The housing of the differential can not rotate in two directions. A nice explanation of this construction, that is also used in LEGO clocks (the "cogwheel"), is given by Robert Caillot
These two pictures show details of the mechanism that stops the wheel. The wheel is released as soon as a ball falls into the "basket"
Today, 6-10-203, I have discovered a site that is completely devoted to marble tracks. Check Jelles Knikkerbaan Further, I have found another way to lift marbles. Check Matthias's marble pump!