As railways developed and expanded, one of the key issues was the track gauge (the distance, or width, between the inner sides of the rail heads) to be used, as the wheels of the rolling stock (locomotives, cars, etc.)
The result was the adoption throughout a large part of the world of a "standard gauge" of 1,435 mm (4 ft 8+1⁄2 in), allowing interconnectivity and interoperability.
A popular legend that has circulated since at least 1937[8] traces the origin of the 1,435 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.
'"[10] The historical tendency to place the wheels of horse-drawn vehicles around 5 ft (1,524 mm) apart probably derives from the width needed to fit a carthorse in between the shafts.
[10] 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".
[12][13] English railway pioneer George Stephenson spent much of his early engineering career working for the coal mines of County Durham.
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 extra half inch was not regarded at first as very significant, and some early trains ran on both gauges daily without compromising safety.
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 continental Europe, France and Belgium adopted a 1,500 mm (4 ft 11+1⁄16 in) gauge (measured between the midpoints of each rail's profile) for their early railways.
The first tracks in Austria and in the Netherlands had other gauges (1,000 mm or 3 ft 3+3⁄8 in in Austria for the Donau Moldau line and 1,945 mm or 6 ft 4+9⁄16 in 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.
[citation needed] 2,295 km (1,426 mi) Victoria built the first railways to the 5 ft 3 in (1,600 mm) Irish broad gauge.