What is the standard gauge of most high-speed rail tracks?

The standard gauge of most high-speed rail tracks is 1,435 millimetres.

This measurement, commonly written as 1,435 mm or 4 feet 8½ inches, is called standard gauge. It is the dominant track gauge for high-speed rail because many major systems—including Japan’s Shinkansen, France’s TGV, Spain’s newer high-speed lines, and China’s high-speed network—use it.

Gauge is the distance between the inner faces of the two rails. It is not the same as the width of the train, the spacing between tracks, or the rail’s loading gauge, which describes the maximum vehicle dimensions permitted on a route.

The answer is “most,” not “all.” Spain’s older high-speed network famously used standard gauge even though much of its conventional railway uses Iberian gauge, while some high-speed lines elsewhere use different gauges for historical or engineering reasons. Japan’s conventional network is largely narrow gauge, but its dedicated Shinkansen lines use standard gauge.