Track gauge is the distance between the inner sides of the rails.

Standard Gauge
When the English began building railways, they adopted the "track gauge" from the horse-drawn wagons of the time. When English railway technology was exported worldwide, standard gauge followed suit. It became standard in Europe (with some exceptions), North America, North Africa, China, and parts of Australia.

Narrow Gauge
The historical roots of narrow-gauge railways trace back to vehicles where the pulling force was not provided by horses but by humans. On these, the track gauge could be as narrow as 600 millimetres. The advantage of narrow-gauge railways was that tracks could be built more easily, with steeper gradients, tighter curves, and smaller, cheaper rolling stock than on standard gauge. The disadvantage was that trains became slower.

Track Gauges in Sweden
All railways built by SJ were standard gauge. However, many of the private railways were narrow gauge. The most widespread narrow gauge in Sweden was 891 millimetres (with networks in Småland-Öland-Östergötland, Västergötland, Roslagen, and Gotland). Next came the gauge of 1067 millimetres (with a large contiguous network in Skåne-Blekinge-southern Småland-Halland). Track gauges of 600, 802, 1093, 1101, and 1217 millimetres have also occurred on Swedish railways.

Narrow-gauge railways are either closed down or converted to standard gauge today. Only the Roslagsbanan still operates narrow gauge (891 millimetres) in daily service. Additionally, there are several narrow-gauge museum railways.

On the wall, there are examples of standard gauge, 1435 millimetres, and the most common Swedish track gauges of 1067, 891, and 600 millimetres.

Track Gauges Worldwide
Exceptions to standard gauge in Europe include Ireland, Spain, Portugal, Finland, and the former Soviet Union, all of which have broader gauges. In Africa, large parts of Australia, Japan, India, and other countries in Asia, the standard track gauge is 1000 millimetres or 1067 millimetres. In other parts of Australia, Asia, and Latin America, it's mixed; broad, standard, and narrow gauges coexist.