Track gauge

Track gauge is the spacing of the rails on a railway track and is measured between the inner faces of the load-bearing rails. All rail vehicles on a network must have running gear that is compatible with the track gauge. As the dominant parameter determining interoperability, it is still frequently used as a descriptor of a route or network.

Today 60% of the world's railways use a gauge of 1,435 mm (4 ft 8 1⁄2 in), known as standard or international gauge. The gauge wider than standard gauge is called broad gauge. The gauge that narrower than standard gauge is called narrow gauge.

Some stretches of track are dual gauge, with three (or sometimes four) parallel rails in place of the usual two, to allow trains of two different gauges to use the same track.

Rolling stock on the network must have running gear (wheelsets) that are compatible with the gauge. This is because the gauge is a one of the key parameter in determining interoperability of rail transport system.

Standard gauge

The terms standard gauge, broad gauge and narrow gauge do not have any fixed meaning.

A "standard" gauge is only standard in a geographical region where it is dominant, but it is generally understood to be 1,435mm (4ft 8½ in).

An infrastructure owner would be ill-advised to order track materials simply as "standard gauge", but would normally specify the required critical dimensions of the components.

Narrow gauge

The most widely used narrow gauges on public railways are 1,067 mm (3 ft 6 in) (Southern and Central Africa, Indonesia, Japan, Taiwan, Philippines, parts of Australia, New Zealand, Honduras and Costa Rica.)

1,000 mm (3 ft 3 3/8in) meter gauge (South East Asia, 17,000 km (11,000 miles) in India, East Africa, South America and Central Europe).

762 mm (2 ft 6 in) (formerly in Sri Lanka Kelani Valley Line and Udapussellawa lines).

Broad gauge

Wider gauges provide greater stability.
Eg. The Great Western Railway (GWR) adopted a gauge of 7 ft (2,134 mm), later eased to 7 ft 1⁄4 in (2,140 mm).
Nowadays, many new rail infrastructures design the track with a wider gauge to ensure safety for high-speed rail operation.

Break of gauge

Rail operation between railway networks with different gauges was originally impossible; goods had to be transhipped and passengers had to change trains.

On narrow gauge lines, Rollbocks or transporter wagons are used: standard gauge wagons are carried on narrow gauge lines on these special vehicles, generally with rails of the wider gauge to enable those vehicles to roll on and off at transfer points.

A system developed by Talgo and Construcciones y Auxiliar de Ferrocarriles (CAF) of Spain uses variable gauge wheelsets; at the border between France and Spain, passenger trains are drawn slowly through apparatus that alters the gauge of the wheels, which slide laterally on the axles. This is fully described in Automatic Gauge Changeover for Trains in Spain.

A similar system is used between China and Central Asia, and between Poland and Ukraine, using the SUW 2000 and INTERGAUGE variable axle systems. China and Poland use a standard gauge, while Central Asia and Ukraine use 1,520 mm (4 ft 11 27⁄32 in).

Mixed gauge

A mixed gauge is where a railway corridor is used by trains of two gauges. Therefore, mixed gauge (or dual gauge) track can be provided, in which three rails are supported in the same track structure.

This arose particularly when individual railway companies chose different gauges and were subsequently required to share a route; this is most commonly found at the approaches to city terminals, where land space is limited.

Trains of different gauges sharing the same track can save considerable expense compared to using separate tracks for each gauge, however, it introduces complexities in track maintenance and signalling system, and may require speed restrictions for some trains.

If the difference between the two gauges is large enough, for example between 1,435 mm (4 ft 8 1⁄2 in) and 3 ft 6 in (1,067 mm), three-rail dual-gauge is possible, but if not, for example between 3 ft 6 in (1,067 mm) and 1,000 mm (3 ft 3 3⁄8 in) meter gauge, four-rail triple-gauge is used.

Dual-gauge rail lines are used in Switzerland, Australia, Argentina, Brazil, Japan, North Korea, Spain, Tunisia and Vietnam.

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