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Coupler height
Use a dedicated coupler-height gauge. A small vertical mismatch can cause unwanted uncoupling, especially over grades and vertical curves.
Wheel gauge
Check wheelsets with the appropriate standards gauge for your scale. Wheels that are too narrow or too wide can pick turnout points and frogs.
Truck movement
Trucks should rotate freely. A common approach is to let one truck rock slightly for three-point suspension while the other is somewhat more controlled, but the best setup depends on the car design.
Car weight
Consistent weight can improve tracking, but excessive weight increases train resistance. Follow an established scale practice and keep the fleet reasonably consistent.