This machine is a type of simple transmission made entirely out of Lego's.  It is capable of operating at three different gear ratios (1:3, 1:1, 3:1).  This model is more of a "proof-of-concept" device and does not include any motors, sensors, or a functional steering mechanism.  However, it could be easily modified to accommadate various other components.  The way it works is by first transmitting power from the top shaft to the bottom shaft.  This is done by means of a special gearing system that ensures that the two shafts are constantly engaged throughout all the machine's various gear configurations.  The bottom shaft is able to move back and forth to engage the top shaft with different gears.  This horizontal movement is controlled by a rack gear attached to a worm gear reduction gearbox (a motor would be attached to the reduction gearbox in a completed machine).  The two sections of the top shaft are not connected, which allows the sections to rotate at different speeds.  The power is transmitted from the top shaft to the wheels by a simple systems of gears attached to a differential gearbox.  If I get enough requests, I may consider making a functional version with the motors and RCX.   However, I worry that the Lego motors would not be able to handle the strain of switching gears at higher speeds.