Track gauge conversion is the changing of one railway track gauge to another. In general, requirements depend on whether the conversion is from a wider gauge to a narrower gauge or vice versa, on how
Convertible concrete sleepers in Spain prefitted for Iberian gauge and standard gauge, with rail fasteners currently in Iberian-gauge position.
In Australia, the multiplicity of track gauges prompted locomotive builders, after the early 1920s, to provide for a potential change of gauge. Three classes of large locomotives ordered by the South Australian Railways in 1924, including the 600 class pictured here (right), were configured to permit easy conversion from 1600 mm (5 ft 3 in) broad gauge to 1435 mm (4 ft 8+1⁄2 in) standard gauge. The older Rx class locomotive next to it was incapable of being converted because its frames were too wide for standard gauge.