Permanent Way
The temporary way is so called because it is the temporary track often used for construction purposes which is replaced by the Permanent Way (the structure consisting of the rails, fasteners, sleepers/ties and ballast (or slab track), plus the underlying subgrade) when construction nears completion. In many cases a narrow gauge track was used for a temporary way because of the convenience in laying it and changing its location over unimproved ground.
In restricted spaces such as tunnels, the temporary way might be double track, even though the tunnel will ultimately be single track. Thus the Airport Rail Link in Sydney had construction trains of 900 mm (2 ft 11 7⁄16 in) gauge which were replaced by the permanent tracks of 1,435 mm (4 ft 8 1⁄2 in) gauge.
During World War I trench warfare had led to a relatively static disposition of infantry forces requiring considerable logistics to bring them support staff and supplies (food, ammunition, earthworks materials, etc.). Dense light railway networks using temporary narrow gauge track sections were established by both sides for this purpose.
During World War II, it was proposed to expedite a Yunnan-Burma railway using a gauge of 1 ft 3.3in, since such a small gauge facilitates the tightest of curves in difficult terrain.
Read more about this topic: Rail Gauge, Temporary Way
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