A system of gears which transmits motion from one shaft to another.This moves an oscillating weight that drives a rotor via a tiny gear train....
The first things to go in were the engines, a job that required a lot of measuring and development of a gear train to match their output to the belts and shafts that drove the looms.
Also, the gear train of a mechanical watch is under constant load from the mainspring whereas the gear train in a quartz watch is not under any load.