Track gauge conversion
In rail transport, gauge conversion is the process of converting a railway from one rail gauge to another, through the alteration of the railway tracks. An alternative to gauge conversion is dual gauge track, or gauge conversion of the rail vehicles themselves.
Ideally railways should all be built to the same gauge, since a wide range of gauges from narrow to broad are of similar value in carrying heavy loads at low cost, while small differences of gauge create tremendous break-of-gauge costs and inconvenience.
Rails may be too light for the loads imposed by broader-gauge railcars and need to be replaced with a heavier rail profile.
If the conversion involves the track gauge being narrowed, the existing sleepers can often continue to be used, but if the gauge is being increased, the sleepers used for the narrower gauge may be too short, and need to be replaced with longer ones.
In some cases, gauge-convertible sleepers are installed before the conversion of the track itself. The sleepers must be long enough to take the wider of the two gauges, and must be able to accommodate the fittings for both the existing and the new gauges. In cases where the difference between the two gauges is small, such as 1,000 mm (3 ft 3 3⁄8 in) and 1,067 mm (3 ft 6 in) track, or 1,435 mm (4 ft 8 1⁄2 in) and 1,524 mm (5 ft) track, providing dual-gauge track using a third rail is not practicable. In those cases, gauntlet track is required.
During the conversion of the Melbourne–Adelaide railway in Australia from 5 ft 3 in (1,600 mm) to 4 ft 8 1⁄2 in (1,435 mm), dual-gauging of the track was not possible because the foot of the heavy rails being used was too wide to allow them to be placed sufficiently close together. A special gauge-convertible sleeper, with a reversible chair for the Pandrol clip, allowed a two-week conversion process.
...
Wikipedia