Rail gauge is the distance between the inner sides of the two parallel rails which make up a railway track. Sixty percent of the world's railways use a gauge of 1435 mm (4 ft 8½ in), which is known as the standard or international gauge. Rail gauges wider than standard gauge are called broad gauge, and rail gauges smaller than standard are called narrow gauge. Some stretches of track are built to a dual gauge: that is to say that three (or sometimes four) parallel running-rails are laid in place of the usual two, in order to allow trains of two different gauges to share the same route. The term break-of-gauge refers to the situation obtaining at a place where different gauges meet.

History
Standard gauge was developed by the British engineer George Stephenson, designer of the Stockton and Darlington Railway, who convinced manufacturers to build equipment using the 4 ft 8½ inch (1435 mm) standard. In 1845 a royal commiion recommended adoption of the standard gauge, and in the following year Parliament passed the Gauge Act, which required that new railways use standard gauge. Except for the Great Western Railway's broad gauge, few main-line British railways used a different gauge, and the Great Western was finally converted to standard gauge in 1892.

Originally a variety of gauges was used in the United States and Canada. Some, primarily in the north-east, used standard gauge; others did not, including track gauges of up to 6 ft (1829 mm). Given the nation's recent independence from the United Kingdom, arguments based on British standards had little weight. Problems began as soon as railroads began to break of gauge became a major economic nuisance. After considerable debate and planning, most of the southern rail network was converted from 5 ft (1524 mm) gauge to 4 ft 9 in (1448 mm) gauge, then the standard of the Pennsylvania Railroad, over two remarkable days beginning on May 31, 1886. The final conversion to standard gauge took place gradually as track was maintained.

In the nineteenth century, Russia chose a broader gauge. It is widely believed that the choice was made for military reasons, to prevent potential invaders from using the Russian rail system. Others point out that no clear standard had emerged by 1842. Engineer Pavel Melnikov hired George Washington Whistler, a prominent American railroad engineer (and father of the artist James McNeill Whistler), to be a consultant on the building of Russia's first major railroad, the Moscow - St Petersburg line. The selection of 1.5 m gauge was recommended by German and Austrian engineers, it was not the same as the ongolia, have the official Russian measurement of 1520 mm, 4 mm narrower than 5 ft (1524 mm), though rolling stock of both gauges is interchangeable in practice.Finland, which was a "Grand Duchy" under Russia in the 19th century, uses 5 ft (1524 mm) gauge. Upon gaining independence in 1917, much thought was given to converting to standard gauge, but nothing came of it.

The main railway networks of Spain and Portugal were originally constructed to gauges of six Castilian feet and five Portuguese feet. Later, following adoption of the metric system, these two gauges were defined as 1674 mm (5 ft 5.9 in) and 1665 mm (5 ft 5.55 in) respectively. The gauges were sufficiently close to allow inter-operation of trains, and in recent years both have been adjusted to a common Iberic gauge of 1668 mm. Since the beginning of the 1990s, however, new high-speed passenger lines in Spain have been built to the international standard gauge of 1435 mm, since it is intended that thesy, Australia's then three mainland states originally adopted standard gauge, but due to political differences a break of gauge 30 years in the future was created. After instigating a change to 5 ft 3 inch (1600 mm) agreed to by all, New South Wales reverted back to standard gauge while Victoria and South Australia stayed with 5 ft 3 in (1600 mm) gauge. Ireland and Northern Ireland also use 1600 mm gauge. Queensland, Tasmania, Western Australia and parts of South Australia adopted the supposedly cheaper narrow gauge 3 ft 6 in (1067 mm).
In Toronto, Canada the Toronto Transit Commission subways and streetcars use 4 ft 10 7/8 in (1495 mm) gauge, making their equipment incompatible with all other city transit systems. The Bay Area Rapid Transit system in the San Francisco Bard gauge only at low speeds. A new railway line across the Hong Kong-Zhuhai-Macau Bridge, an extension to the 1432 mm gauge Tung Chung Line, is a future option, but one which would require solving the problem of the 3 mm difference.

On independence from Britain, India, Pakistan, Bangladesh and Sri Lanka inherited a diversity of rail gauges, of which 1676 mRailways' narrower gauge railways to the 1676 mm. Argentina and Chile also have 1676 mm lines.
Afghanistan is in an interesting position, because it is at the crossroads of Asia and is virtually without railways. Should it decide to build any, the choice of gauge will be complicated by its being surrounded by three different gauges (1435 mm, 1520 mm, 1676 mm) in four "gauge oceans" ? Iran to the west uses standard gauge, as does China to the east; to the sout, Pakistan uses 1676 mm gauge, while to the north, the central Asian republics of Turkmenia, Uzbekistan, and Tajikistan use 1520 mm gauge.

In many areas a much narrower gauge was chosen. While narrow gauge generally cannot handle as much tonnage, it is less costly to ce-gauge (1000 mm). The proposed ASEAN railway, would be a standard-gauge or dual-gauge (metre and standard gauge) regional railway network, linking Singapore, at the southern tip of the Malay Peninsula, through Malaysia, Thailand, Laos and Vietnam to China's standard-gauge railway railways are built to 3 ft 6 in (1067 mm) gauge. Japan's high-speed Shinkansen, or bullet trains are built to standard gauge for greater stability, which has caused some difficulties integrating high-speed and conventional passenger railway services. Taiwan, which was ruled by Japan from 1895 to 1945, has several 1067 mm lines, although its HSR high-speed line will also be standard gauge. The railways of South Africa and many other African countries, including Angola, Botswana, Congo, Ghana, Mozambique, Namibia, Nigeria, Zambia and Zimbabwe, use 1067 mm gauge, which is sometimes referred to as Cape gauge. Indonesia's railways are predominantly 1067 mm.

Dual gauge and adjustable axles
Dual gauge allows trains of different gauges to share the same right of way. This can save considerable expense compared to using separate tracks for each gauge. There can, however, be many difficulties and speed restrictions. If the difference between the two gauges is enough, three-rail dual-gauge is possible (say 1435 mm and 1067 mm), but if the difference is not enough, four-rail dual-gauge is necessary (say 1067 mm and 1000 mm). Dual gauge helps the conversion from one gauge to another. Dual-gauge rail lines are used in sections of the railway networks of Switzerland, Australia, auge Axles developed by the Talgo company in Spain enable trains to travel from broad to standard gauge with only a few minutes spent in the gauge conversion process. The same system is also used between China (standard gauge) and Central Asia (1524 mm). It is not known why the VGA system is not more widely used, particularly between standard and narrow gauges.

Future
Further standardization of rail gauges seems likely, as individual countries seek to build inter-operable national networks, and international organizations seek to build macro-regional and even continental networks. National projects include the Australian and Indian efforts mentioned above to create a uniform gauge in their national networks. The European Union has set out to develop inter-operable freight and passenger rail networks across the EU area, and is seeking to standardize not only track gauge, but also signaling and electrical power systems. EU funds have been dedicated to convert key railway lines in the Baltic states of Lithuania, Latvia, and Estonia from the 1520 mm gauge to standard gauge, and to assist Spain and Portugal in the construction of additional high-speed rail lines to connect Iberian cities to one another and to the French high-speed lines. The EU has also developed plans for improved freight rail links between Spain, Portugal, and the rest of Europe.

All high-speed rail systems around the world have been built using or planning to use standard gauge, even in countries like Japan, Taiwan, Spain and Portugal where most of the country's existing rail lines use a different gauge. Once standard gauge high-speed networks exist, they may provide the impetus for gauge conversion of existing passenger lines to allo United Nations Economic and Social Commission for Asia and the Pacific (UNESCAP) is planning a Trans-Asian Railway that will link Europe and the Pacific, with a Northern Corridor from Europe to the Korean Peninsula, a Southern Corridor from Europe to Southeast Asia, and a North-South corridor from Northern Europe to the Persian Gulf. All the proposed corridors would encounter one or more breaks of gauge as they cross Asia. Current plans do not call for widespread gauge conversion; instead, mechanized facilities would be built to move shipping containers from train to train at the breaks of gauge.

Kenya-Uganda-Sudan proposal
A proposal was aired in October 2004 to built a high speed electrified line to connect Kenya with southern Sudan. Kenya and Uganda have 1000 mm gauge, while Sudan is 1067 mm - by choosing standard gauge for the project, the gauge incompatibility is overcome. A bonus would be that standard gauge matches that used by Egypt further north. Since the existing narrow gauge track is quite likely of a pioneer standard (with sharp curves and low capacity light rails) substanly origins of the standard gauge
There is a story that rail gauge was derived from the rutways created by war chariots used by Imperial Rome, which everyone else had to follow to preserve their wagon wheels, and because Julius Caesar set this width under Roman law so that vehicles could traverse Roman villages and towns without getting caught in stone ruts of differing widths. However, an equal gauge is probably coincidence. Excavations at the buried cities of Pompeii and Herculaneum revealed ruts averaged 4 ft 9 in (1448 mm) center to center, with a gauge of 4 ft 6 in (1372 mm). The designers of both chariots and trams and trains were dealing with a similar issue, namely hauling wheeled vehicles behind draft animals.

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