Standard Gauge

The Standard Gauge, also known as the International Gauge or UIC Gauge, is the Gauge used by over half of the Railways in the world today.
The Gauge is 4 feet 8 and a half inches (1,435 mm) measured from inside rail to inside rail.

It was the Gauge introduced by George Stephenson on his early Railways.
Stephenson had already built horse drawn Railways in the Collieries to a Gauge of 4 feet 8 inches, with hundreds of trucks to move coal. So the Stockton and Darlington railway was built to the same Gauge.
After 15 years he extended the Gauge to 4 feet 8 and a half. The extra half an inch was to help prevent binding on curves.
In 1846, The Gauge Act determined all new railways in Great Britain should be built to Standard Gauge.

The Gauge of the Colliery trucks was 4 foot 8 inches because it was based on the cart ruts in the Roman Roads.
The Cart Ruts in Pompeii have been measured at approximately 4 feet 9 inches.
This was the width that allowed two horses to be harnessed together and pull a cart.
If Medieval Carts didn’t fit the Ruts in Roman Roads, they risked breaking a wheel.
So most Medieval Carts were built to the same gauge as Roman Carts, a tradition that carried on up to the nineteenth century CE.

During the Cold War, the LGM 118A Peacekeeper Missile Program called for 50 Missiles to be launched from Railcars. They were dispersed along the Rail system, in order to prevent a Soviet pre-emptive strike on fixed Missile Silos.
They were designed to fit into the Boxcars that travelled on a gauge of 4 feet 8 and a half inches. Although the missiles themselves were wider, their compact design was influenced by the Roman Cart Gauge.