Geared steam locomotive
A geared steam locomotive is a type of steam locomotive which uses gearing, usually reduction gearing, in the drivetrain, as opposed to the common directly driven design.
Lego Technic
Gears have been included within Lego Technic sets since 1977 as a way of transferring rotary power, and of gearing-up or down the speed. Gears come in several sizes: 8 tooth, 16 tooth, 24 tooth and 40 tooth spur gears; 12 tooth, 20 tooth and 36 tooth double bevel gears; and 12 tooth and 20 tooth single bevel gears. The double bevel gears are cut so they can also be meshed as spur gears. There is also a 16 tooth clutch gear, and a 24 tooth friction gear that slips when a certain amount of torque is put on it to prevent motors from damaging any parts or burning themselves out.