Configuration
For dual-gauge track to be achievable using three rails, the difference between the gauges needs to be at least as wide as the foot of the rail, otherwise there is no room for the rail fastening hardware (spikes, clips, and the like). Thus standard gauge (1,435 mm (4 ft 8 1⁄2 in)) and Indian gauge 1,676 mm (5 ft 6 in) can be dual gauged without problem, as are 914 mm (3 ft) and 1,067 mm (3 ft 6 in) (Cape gauge), or 1,524 mm (5 ft) and 1,829 mm (6 ft), while 1,435 mm (4 ft 8 1⁄2 in) and 1,600 mm (5 ft 3 in) (Irish gauge) can also be dual-gauged, albeit with lighter narrow footed rails, as in Victoria, Australia. On the other hand, metre gauge 1,000 mm (3 ft 3 3⁄8 in) and cape gauge 1,067 mm (3 ft 6 in) as in Africa, or 1,000 mm (3 ft 3 3⁄8 in) and 3 ft (914 mm), as in South America, are too close to be combined into three-rail dual gauge, as are 1,435 mm (4 ft 8 1⁄2 in) and 1,524 mm (5 ft), or 1,524 mm (5 ft) and 1,676 mm (5 ft 6 in), as in Afghanistan, Central Asia, northern-, central- and eastern Europe, North America, Iran, and China. This last combination is of particular historical interest, as it was of strategic significance during World War II.
If three-rail dual gauge is impossible (e.g. between 1,435 mm (4 ft 8 1⁄2 in) and 1,524 mm (5 ft) ), four-rail dual gauge has to be used.
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