Gauge is the distance between the two running rails. This is measured between the “gauge point” on the rails, which is at a distance of typically 12-14mm (depending on the network) below the reference line that is formed by a straight edge lying on the crown of both running rails in track.
The high rail is the outer rail in a curve and the low rail is the inner rail. The difference in height is the cant, which compensates partially for curving forces. The equilibrium cant required for a vehicle on standard gauge track (distance of 1.52m between rail centres) to travel through a curve of radius R (m) at speed v (km/h) is
In straight track, wheels should ideally run along the rail crown. Contact between wheels and the gauge face should occur only in severe curves. Contact occurs on the gauge shoulder in most curves, since this gives rise to a rolling radius difference between wheels on the same wheelset, thereby helping it to roll around the curve.