Standard-gauge railway

A standard-gauge railway is a railway with a track gauge of. The standard gauge is also called Stephenson gauge (after George Stephenson), international gauge, UIC gauge, uniform gauge, normal gauge in Europe,    and SGR in East Africa. It is the most widely used track gauge around the world, with about 55% of the lines in the world using it.

All high-speed rail lines use standard gauge except those in Russia, Finland, and Uzbekistan. The distance between the inside edges of the rails is defined to be 1,435 mm except in the United States, Canada, and on some heritage British lines, where it is defined in U.S. customary/Imperial units as exactly "four feet eight and one half inches", which is equivalent to 1,435.1mm.

History
As railways developed and expanded, one of the key issues was the track gauge (the distance, or width, between the inner sides of the rails) to be used. Different railways used different gauges, and where rails of different gauge met – a "gauge break" – loads had to be unloaded from one set of rail cars and reloaded onto another, a time-consuming and expensive process. The result was the adoption throughout a large part of the world of a "standard gauge" of, allowing interconnectivity and interoperability.

Origins
A popular legend that has circulated since at least 1937 traces the origin of the gauge even further back than the coalfields of northern England, pointing to the evidence of rutted roads marked by chariot wheels dating from the Roman Empire. Snopes categorised this legend as "false", but commented that it "is perhaps more fairly labeled as 'Partly true, but for trivial and unremarkable reasons. The historical tendency to place the wheels of horse-drawn vehicles around apart probably derives from the width needed to fit a carthorse in between the shafts. Research, however, has been undertaken to support the hypothesis that "the origin of the standard gauge of the railway might result from an interval of wheel ruts of prehistoric ancient carriages".

In addition, while road-travelling vehicles are typically measured from the outermost portions of the wheel rims, it became apparent that for vehicles travelling on rails, having main wheel flanges that fit inside the rails is better, thus the minimum distance between the wheels (and, by extension, the inside faces of the rail heads) was the important one.

A standard gauge for horse railways never existed, but rough groupings were used; in the north of England none was less than. Wylam colliery's system, built before 1763, was, as was John Blenkinsop's Middleton Railway; the old plateway was relaid to  so that Blenkinsop's engine could be used. Others were (in Beamish) or  (in Bigges Main (in Wallsend), Kenton, and Coxlodge).

English railway pioneer George Stephenson spent much of his early engineering career working for the coal mines of County Durham. He favoured   for wagonways in Northumberland and Durham, and used it on his Killingworth line. The Hetton and Springwell wagonways also used this gauge.

Stephenson's Stockton and Darlington railway (S&DR) was built primarily to transport coal from mines near Shildon to the port at Stockton-on-Tees. Opening in 1825, the initial gauge of was set to accommodate the existing gauge of hundreds of horse-drawn chaldron wagons that were already in use on the wagonways in the mines. The railway used this gauge for 15 years before a change was made, debuting around 1850, to the gauge. The historic Mount Washington Cog Railway, the world's first mountain-climbing rack railway, is still in operation in the 21st century, and has used the earlier gauge since its inauguration in 1868.

George Stephenson introduced the gauge (including a belated extra 1/2 in of free movement to reduce binding on curves) for the Liverpool and Manchester Railway, authorised in 1826 and opened 30 September 1830. The extra half inch was not regarded at first as very significant, and some early trains ran on both gauges daily without compromising safety.

The success of this project led to Stephenson and his son Robert being employed to engineer several other larger railway projects. Thus the gauge became widespread and dominant in Britain. Robert was reported to have said that if he had had a second chance to choose a gauge, he would have chosen one wider than. "I would take a few inches more, but a very few".

During the "gauge war" with the Great Western Railway, standard gauge was called "narrow gauge", in contrast to the Great Western's broad gauge. The modern use of the term "narrow gauge" for gauges less than standard did not arise for many years, until the first such locomotive-hauled passenger railway, the Ffestiniog Railway, was built.

Adoption
In 1845, in the United Kingdom of Great Britain and Ireland, a Royal Commission on Railway Gauges reported in favour of a standard gauge. The subsequent Gauge Act ruled that new passenger-carrying railways in Great Britain should be built to a standard gauge of, and those in Ireland to a new standard gauge of. In Great Britain, Stephenson's gauge was chosen on the grounds that existing lines of this gauge were eight times longer than those of the rival (later ) gauge adopted principally by the Great Western Railway. It allowed the broad-gauge companies in Great Britain to continue with their tracks and expand their networks within the "Limits of Deviation" and the exceptions defined in the Act.

After an intervening period of mixed-gauge operation (tracks were laid with three rails), the Great Western Railway finally completed the conversion of its network to standard gauge in 1892. In North East England, some early lines in colliery (coal mining) areas were, while in Scotland some early lines were. The British gauges converged starting from 1846 as the advantages of equipment interchange became increasingly apparent. By the 1890s, the entire network was converted to standard gauge.

The Royal Commission made no comment about small lines narrower than standard gauge (to be called "narrow gauge"), such as the Ffestiniog Railway. Thus it permitted a future multiplicity of narrow gauges in the UK. It also made no comments about future gauges in British colonies, which allowed various gauges to be adopted across the colonies.

Parts of the United States, mainly in the Northeast, adopted the same gauge, because some early trains were purchased from Britain. The American gauges converged, as the advantages of equipment interchange became increasingly apparent. Notably, all the broad gauge track in the South was converted to "almost standard" gauge  over the course of two days beginning on 31 May 1886. See Track gauge in the United States.

In continental Europe, France and Belgium adopted a gauge (measured between the midpoints of each rail's profile) for their early railways. The gauge between the interior edges of the rails (the measurement adopted from 1844) differed slightly between countries, and even between networks within a country (for example, to  in France). The first tracks in Austria and in the Netherlands had other gauges ( in Austria for the Donau Moldau line and in the Netherlands for the Hollandsche IJzeren Spoorweg-Maatschappij), but for interoperability reasons (the first rail service between Paris and Berlin began in 1849, first Chaix timetable) Germany adopted standard gauges, as did most other European countries.

The modern method of measuring rail gauge was agreed in the first Berne rail convention of 1886.

Almost standard gauge

 * The Huddersfield Corporation Tramways, used
 * The Portsdown and Horndean Light Railway, used
 * The Portsmouth Corporation Transport, used
 * The Killingworth colliery railway, used.
 * The Hetton colliery railway, opened 1822, used.
 * The Stockton and Darlington Railway, authorised 1821, opened 1825, used.
 * The New Orleans and Carrollton Railroad used
 * The Pontchartrain Railroad used
 * The trams in Nuremberg nominally used during much of their existence, but have since been converted to standard gauge in name as well as fact.

Small deviations from standard gauge

 * The Manchester and Leeds Railway, authorised on 4 July 1836, used.
 * The railways were intended to take  gauge vehicles and allow a (second) running tolerance.
 * The Chester and Birkenhead Railway, authorised on 12 July 1837, used.
 * The London and Brighton Railway, authorised on 15 July 1837, used.
 * The Manchester and Birmingham Railway, authorised on 30 June 1837, used.
 * The Pennsylvania Railroad originally used
 * The trams in Dresden, authorised in 1872 as horsecars, used gauge vehicles. Converted to 600 V DC electric trams in 1893, they now use ; both gauges are within the tolerance for standard gauge.
 * The Ohio gauge of

Dual gauge

 * Cheltenham and Great Western Union Railway, authorised 1836, opened 1840, dual gauge 1843 and.

Initially standard gauge
Several lines were initially built as standard gauge but were later converted to another gauge for cost or for compatibility reasons.
 * South Africa became
 * Thailand became
 * Indonesia became
 * Ireland became – Dublin and Kingstown Railway
 * Australia became – Victoria & South Australia – partly converted to
 * India became – initial freight lines
 * some private Japanese railways

Modern almost standard gauge railways

 * The Toronto Transit Commission uses a Toronto gauge of on its streetcar and heavy-rail subway lines, which was actually closer to  gauge.
 * The Toronto Transit Commission light-metro lines and light-rail lines (whether existing, under construction or proposed) use standard gauge.
 * Trams in Leipzig, Germany use.
 * Trams in Dresden, Germany use.
 * gauge is in use on several urban rail transit systems in Europe:
 * Trams in Italy
 * Madrid Metro (only metro system. Light rail system uses standard gauge.)
 * The MTR in Hong Kong uses gauge on lines owned by the MTR Corporation. However, lines formerly operated (but which continue to be owned) by the Kowloon-Canton Railway Corporation, including the Light Rail network, use  gauge. New lines and extensions to the MTR after 2014 use  gauge, including the South Island line, Kwun Tong line extension and West Island line.
 * The Bucharest Metro uses gauge.
 * The Washington Metro uses Washington Metro rolling stock, 1/4 in narrower than standard gauge.
 * The Mount Washington Cog Railway, the world's oldest mountain-climbing rack-and-pinion railway, uses a gauge.

Non-rail use
Several states in the United States had laws requiring road vehicles to have a consistent gauge to allow them to follow ruts in the road. Those gauges were similar to railway standard gauge.