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 re-loaded 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 1435 mm (4 ft 8 1⁄2 in), allowing interconnectivity and interoperability.
A popular legend that has been around since at least 1937 traces the origin of the 1435 mm (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.[a] It is curious that the Roman pace or passus was 4.855 ft or 1435 mm; a thousand such was one Roman mile. Snopes categorised 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 feet (1,500 mm) apart probably derives from the width needed to fit a carthorse in between the shafts. In addition, while road-travelling vehicles are typically measured from the outermost portions of the wheel rims (and there is some evidence that the first railways 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 never a standard gauge for horse railways, but there were rough groupings: in the north of England none was less than (1,219 mm). Wylam colliery's system, built before 1763, was 5 ft (1,524 mm), as was John Blenkinsop's Middleton Railway; the old 4 ft (1,219 mm) plateway was relaid to 5 ft (1,524 mm) so that Blenkinsop's engine could be used. Others were 4 ft 4 in (1,321 mm) (in Beamish) or 4 ft 7 1⁄2 (in
Bigges Main (in Wallsend), 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 4 ft 8 in (1,422 mm) 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 4 ft 8 in (1,422 mm) 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 the 4 ft 8 1⁄2 in 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 4 ft 8 in gauge since its inauguration in 1868.
George Stephenson used the 4 ft 8 1⁄2 in gauge (including a belated extra 1⁄2 in (12.7 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 Stephenson and his son Robert being employed to engineer several other larger railway projects. Thus the 4 ft 8 1⁄2 gauge became widespread and dominant in Britain. Robert was reported to have said that if he had had a second chance to choose a standard gauge, he would have chosen one wider than 1,435 mm (4 ft 8 1⁄2 in). "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 7 ft 1⁄4 in 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.
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 1,435 mm (4 ft 8 1⁄2 in), and those in Ireland to a new standard gauge of 5 ft 3 in (1,600 mm). 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 7 ft (2,134 mm) (later 7 ft 1⁄4 in (2,140 mm)) 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 converted its entire network to standard gauge in 1892. In North East England, some early lines in colliery (coal mining) areas were 4 ft 8 in (1,422 mm), while in Scotland some early lines were 4 ft 6 in (1,372 mm). All these lines had been widened to standard gauge by 1846. The British gauges converged starting from 1846 as the advantages of equipment interchange became increasingly apparent. By 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 5 ft (1,524 mm) broad gauge track in the South was converted to 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 1500 mm gauge from axis to axis of rail 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, 1440 to 1445 mm in France).
The first tracks in Austria and in the Netherlands had other gauges (1100 mm in Austria for the Donau Moldau line), 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, according to the "Revue générale des chemins de fer, July 1928".