Standard gauge

The standard gauge (also Stephenson gauge after George Stephenson, UIC (track) gauge, International gauge or normal gauge) is a widely used railway track gauge. Approximately 60% of lines in the world are this gauge (see the list of countries that use the standard gauge). Except for Russia and Finland, all high-speed lines are this gauge.

The distance between the inside edges of the rails is usually called 1,435 mm but in the United States, Canada and Britain it is still called 4 ft $8 1/2$ in.

Standard gauge applies to everyone.

Naming
Standard gauge was originally called "narrow gauge" in contrast to Brunel's broad gauge. When smaller gauges appeared, it was renamed "standard gauge".

It's also called "international gauge", "UIC gauge", or "UIC track gauge"

It's called European gauge in the EU, as well as in Russia.

It's called "uniform gauge" in Queensland.

History
As railways developed and expanded one of the key issues was track gauge (the distance, or width, between the inner sides of the rails) to be used. The result was the adoption throughout a large part of the world of a “standard gauge” of 4 ft $8 1/2$ in allowing inter-connectivity and inter-operability.

In England some early lines in colliery (coal mining) areas in the northeast were ; and in Scotland some early lines were (Scotch gauge). By 1846, in both countries, these lines were widened to standard gauge. Parts of the United States, mainly in the Northeast, adopted the same gauge because some early trains were purchased from Britain. However, until well into the second half of the 19th century Britain and the USA had several different track gauges. The American gauges converged as the advantages of equipment interchange became increasingly apparent; notably, the South's broad gauge was converted to be compatible with standard gauge over the course of two days beginning May 31, 1886. See Track gauge in the United States.

With the advent of metrication, standard gauge was redefined as 1,435 mm, a reduction of 0.1 mm, but well within existing tolerance limits. The exception is the United States, where standard gauge continues to be defined in terms of customary units.

Origins
A popular legend that has been around since at least 1937 traces the origin of the 4 ft $8 1/2$ in 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 categorized this legend as false but commented that “... it is perhaps more fairly labelled as 'True, but for trivial and unremarkable reasons.'" The historical tendency to place the wheels of horse-drawn vehicles approximately 5 ft apart probably derives from the width needed to fit a carthorse in between the shafts. In addition, while road-traveling vehicles are typically measured from the outermost portions of the wheel rims (and there is some evidence that the first railroads were measured in this way as well), it became apparent that for vehicles travelling on rails it was better to have the wheel flanges located inside the rails, and thus the distance measured on the inside of the wheels (and, by extension, the inside faces of the rail heads), was the important one.

There was no standard gauge for horse railways, but there were rough groupings: in the north of England none were 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 Beamish or 4 ft $7 1/2$ in (Bigges Main and Kenton and Coxlodge).

The 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. 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 to 4 ft $8 1/2$ in gauge.

The beginnings of the 4 ft $8 1/2$ in gauge
George Stephenson used the 4 ft $8 1/2$ in gauge (with a belated extra $1/2$ in (13 mm) of free movement to reduce binding on curves ) for the Liverpool and Manchester Railway, authorised in 1826 and opened 30 September 1830. The success of this project led to George Stephenson and his son Robert being employed to engineer several other larger railway projects. However, the Chester and Birkenhead Railway, authorised on 12 July 1837, used ; the Eastern Counties Railway, authorised on 4 July 1836, used ; London and Blackwall Railway, authorised on 28 July 1836, used ; the London and Brighton Railway, authorised on 15 July 1837, used ; the Manchester and Birmingham Railway, authorised on 30 June 1837, used ; the Manchester and Leeds Railway, authorised on 4 July 1836, used and the Northern and Eastern Railway, authorised on 4 July 1836, used  gauge. The railways were intended to take 4 ft $8 1/2$ -gauge vehicles and allow a (second) running tolerance.

The influence of the Stephensons appears to be the main reason that the 4 ft $8 1/2$ in gauge became the standard.

During the "gauge war" with the Great Western Railway, standard gauge was called "narrow gauge". The modern use of narrow gauge for gauges less than standard did not arise for 20 years, until the first such locomotive-hauled passenger railway, the Festiniog.

The Royal Commission
In 1845, in the United Kingdom of Great Britain and Ireland, a Royal Commission reported in favour of a standard gauge. In Great Britain, Stephenson's gauge was chosen on the grounds that lines of this gauge were eight times longer than those of the rival gauge adopted principally by the Great Western Railway. 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. It allowed the broad-gauge companies in Great Britain to continue with their tracks and expanding 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 converted its entire network to standard gauge in 1892.

The Royal Commission made no comment about small to-be-called "narrow"-gauge lines, such as the Festiniog Railway, which allowed a future multiplicity of small gauges in the UK; it also made no comments about future gauges in British colonies.

Regrets
Robert Stephenson was reported to have said that if he had a second chance to choose a standard gauge, he would choose one wider than. "I would take a few inches more, but a very few".

Road vehicles
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. These gauges were similar to railway standard gauge.