What is a gear train and what is its purpose?

A gear train is a combination of two or more gears used to transmit motion and power from one shaft to another. Its purposes include speed reduction or increase (changing RPM), torque multiplication, direction change, and transmitting power between non-coaxial shafts. Types include simple gear trains, compound gear trains, reverted gear trains, and epicyclic (planetary) gear trains used in automobile transmissions and machinery.