Line, top, gauge, superelevation, and twist.
Repeat after me... 'Line, top, gauge, superelevation, and twist.'
Line: Make sure the straights (tangents) are straight, and the curves are curved, no kinks, and the transition between them is smooth.
Top: No sudden dips in the track.
Gauge: 45mm +/- whatever standard you are using. Tight gauge causes wheels to bind.
Superelevation: All good in theory, and on real railways... but don't use it on your garden line.
Twist: No, not the Chubby Checker one, but where one rail is higher than the other, and then the other one is higher. Particualarly on curves.
