Question: Where do you find a simple gear train?
Answer: Automobile drivetrains
transmission
differential
Disambiguated questions and answers: question: What part of machines do you often find a simple gear train?
What non-motorized personal vehicle do you often find a simple gear train?
answer: gearboxes
gearbox
bike
bicycle
cycle
Natural Questions answer: Automobile drivetrains (from the Wikipedia article "Gear train")