^Smiles (1904, p. 160) states that early tramroads had rails 4 ft 8 in (1,422 mm) apart, but Tomlinson (1915, pp. 82–83) challenges this, stating that the most common gauge of the early tramroads and waggonways was about 4 ft (1,219 mm), and some, such as the Wylam waggonway, had the rails 5 ft (1,524 mm) apart. The gauge of the S&DR was given in early documents as 4 ft 8 in (1,422 mm), but the distance between the rails was later measured as 4 ft 8+1⁄2 in (1,435 mm), and this became the standard gauge used by 60 per cent of railways worldwide. The difference of 1⁄2 inch (13 mm) is a mystery.[3][4]
