Track consists of two parallel steel rails, anchored perpendicular to members called sleepers (ties) of timber, concrete, steel, or plastic to maintain a consistent distance apart, or rail gauge. Rail gauges are usually categorized as standard gauge (used on approximately 55% of the world's existing railway lines), broad gauge, and narrow gauge. In addition to the rail gauge, the tracks will be laid to conform with a Loading gauge which defines the maximum height and width for railway vehicles and their loads to ensure safe passage through bridges, tunnels and other structures.