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The rubber-tyred metro systems that incorporate 1,435 mm (4 ft 8 1⁄2 in) standard gauge track have angle irons as guide bars, or guiding bars, outside of the two roll ways. The Busan Subway Line 4, that lacks a rail track, has I-beams installed as guide bars. The flanges are vertical. The Sapporo Municipal Subway, that lacks a rail track as well, has no guide bars. It has a central guide rail instead.