Geared steam locomotive
This gearing is part of the machinery within the locomotive and should not be confused with the pinion that propels a rack locomotive along the rack between the rails. The geared steam locomotives that have been built have been for conventional track, relying on the adhesion between wheels and rail.
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.