The term 'distance between rails' refers to the space between two parallel railroad tracks. This distance is measured in terms of GAUGE, SPAN, and INCH. GAUGE is the distance between the inner edges of the two rails and is typically measured in millimeters or inches. SPAN is the total width of the two rails and is usually measured in feet or meters. INCH is the unit of measurement used to measure the distance between the two rails and is typically measured in inches.