Question: Where do you find a simple gear train?

Disambiguated questions and answers:
context: The simplest transmissions, often called gearboxes to reflect their simplicity (although complex systems are also called gearboxes in the vernacular), provide gear reduction (or, more rarely, an increase in speed), sometimes in conjunction with a right-angle change in direction of the shaft (typically in helicopters, see picture). These are often used on PTO-powered agricultural equipment, since the axial PTO shaft is at odds with the usual need for the driven shaft, which is either vertical (as with rotary mowers), or horizontally extending from one side of the implement to another (as with manure spreaders, flail mowers, and forage wagons). More complex equipment, such as silage choppers and snowblowers, have drives with outputs in more than one direction. So too Helicopters use a split-torque gearbox where power is taken from the engine in two directions for the different rotors. The gearbox in a wind turbine converts the slow, high-torque rotation of the turbine into much faster rotation of the electrical generator. These are much larger and more complicated than the PTO gearboxes in farm equipment. They weigh several tons and typically contain three stages to achieve an overall gear ratio from 40:1 to over 100:1, depending on the size of the turbine. (For aerodynamic and structural reasons, larger turbines have to turn more slowly, but the generators all have to rotate at similar speeds of several thousand rpm.) The first stage of the gearbox is usually a planetary gear, for compactness, and to distribute the enormous torque of the turbine over more teeth of the low-speed shaft. Durability of these gearboxes has been a serious problem for a long time.
question: What part of machines do you often find a simple gear train?
short_answers: gearboxes

gearbox
wikipage: Transmission (mechanics)

context: No context provided
question: What non-motorized personal vehicle do you often find a simple gear train?
short_answers: bike

bicycle

cycle

Long answers:
knowledge: content: A gear train is a mechanical system formed by mounting gears on a frame so the teeth of the gears engage.
wikipage: Gear train

content: Gear teeth are designed to ensure the pitch circles of engaging gears roll on each other without slipping, providing a smooth transmission of rotation from one gear to the next
wikipage: Gear train

content: Geared devices can change the speed, torque, and direction of a power source.
wikipage: Gear

content: Gears of different sizes produce a change in torque, creating a mechanical advantage, through their gear ratio, and thus may be considered a simple machine.
wikipage: Gear
long_answer: A gear train is a mechanical system formed by mounting gears on a frame with rotating, interlocking teeth. The gears can change the speed, torque, and direction of a power source, and are considered a simple machine. Gear trains are used in a variety of machines, and are typically found in the gearbox, which is a form of a simple transmission. You can also find a gear train on non-motorized machines, such a bicycle. 

knowledge: content: [1]A transmission is a machine in a power transmission system, which provides controlled application of power. Often the term 5-speed transmission refers simply to the gearbox, that uses gears and gear trains to provide speed and torque block conversions from a rotating power source to another device.[1][2]
wikipage: Transmission (mechanics)
long_answer: A transmission is a machine in a power transmission system, which provides controlled application of power. Often the term 5-speed transmission refers simply to the gearbox, that uses gears and gear trains to provide speed and torque block conversions from a rotating power source to another device. Simple gear trains are often found in gearboxes on machines as well as on non-motorized bicycles.