This document defines the main wheel/track interaction criteria to be taken into account during the geometrical design of switches and crossings (S&C) layouts. It specifies: characterization of wheel and track dimensions; geometric design principles for wheel guidance; design principles for wheel load transfer; whether movable crossings are needed. These are illustrated by their application to turnout components: switches; crossings; check rails, but the principles apply equally to more complex layouts. There are also simplified definitions of the safety and functional dimensions, which can be used in conjunction with the general principles as the basis for more in-depth assessment.