The gear train is the mechanical backbone of any movement: without understanding it, you cannot understand how a watch measures time. It receives energy from the mainspring, reduces it progressively through a cascade of gears, and delivers it to the escapement with the right speed and torque. Each wheel has an integral pinion: the gear ratio between the wheel's teeth and the pinion's teeth determines the rotation speed of the next stage. The center wheel completes one rotation per hour and drives the hand mechanism; the seconds wheel completes one rotation per minute. The 12:1 reduction system governing the hour hand — the so-called motion work — technically doesn't form part of the main train, but depends directly on it.
In a conventional movement the train is supported by dedicated bridges: the train bridge covers the central group, the escape wheel bridge isolates the escape wheel. The German three-quarter plate solution replaces both with a single extended plate, increasing structural rigidity and protecting the gears. This construction choice substantially changes the visual and mechanical character of the caliber.
Technical variants multiply at the extremes of the market. The flying gear train — as in the Corum LAB 02 — eliminates the upper bridge: the gears remain suspended between barrel and power reserve indicator, replacing jewels with ball bearings and lengthening the axes to compensate for the lack of support. The split gear train divides the path to optimize transmission efficiency. Some calibers, like those of Bernhard Lederer, integrate a constant force mechanism directly into the going train, stabilizing the torque reaching the escapement and measurably improving chronometric precision.
Aesthetically, skeletonization has made the train a visual as well as mechanical element. In the MING 21.01 the bridge is sandblasted titanium with polished angles; in the Rado Captain Cook the golden train bridge becomes the dial itself. The invisible going train of the F.P. Journe Chronomètre Souverain is an extreme case: the train is relocated to the dial side of the plate, eliminating any visible connection between the barrels and the regulator. The optical effect is that of a movement that seems to function without transmission. From the bench I have seen movements with perfectly executed trains in economical mechanisms and neglected trains in expensive cases: the quality of the gears — tolerances, flank finish, axis concentricity — is read in the balance wheel's performance, not in the brand's label.