The motor unit used in all of the monorail sets. The motor is mounted upright and a gear on the motor's drive shaft engages with the toothed rack on the monorail track. There is a bar that sticks out either side of the housing, pushing it one way makes the motor go in one direction, pushing it the other way makes the motor reverse. In the centre position the motor stops. This allows the monorail to be controlled by the #5039 Monorail Stop/Reverse Switch.
When used with #6399 International Shuttle the motor is covered with a special red outer housing, this piece has never been available as a spare part.