Gear trains are combinations of wheels that transmit motion from one shaft to another. There are several types of gear trains including simple, compound, epicyclic, and reverted gear trains. A simple gear train contains one gear on each shaft connected by meshing teeth. An epicyclic or planetary gear train contains one or more outer gears that rotate around a central gear. Gear trains can be used to increase or decrease shaft speed and rotate shafts in the same or opposite directions.