Four Feet, Eight and a Half Inches
Standard gauge railway lines are used throughout New South Wales & on Australian National Railways' interstate lines. It is also the primary gauge used in Britain, Europe, the USA, & many other countries. It is used on such high speed lines as France's TGV, Germany's ICE, & Japan's Bullet Trains.

Standard gauge, in railway terminology, means a distance between the rails of 4 feet, 8 inches or 1.435 metres. That's an exceedingly odd number. Why was that gauge used?

Because that's the way they built them in England, & English expatriates built railways all around the world. Why did the English build them like that?

Because the first railway lines were built by the same people who built the pre-railway tramways, and that's the gauge they used. Why did they use that gauge in England, then?

Because the people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing. Okay! Why did their wagons use that odd wheel spacing?

Because, if they tried to use any other spacing the wagon wheels would break on some of the old, long distance roads. Because that's the spacing of the old wheel ruts. So who built these old rutted roads?

The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The Roman roads have been used ever since. And the ruts?

The original ruts, which everyone else had to match for fear of destroying their wagons, were first made by the wheels of Roman war chariots. Since the chariots were made for or by Imperial Rome they were all alike in the matter of wheel spacing.

Thus, we have the answer to the original question. The standard railway gauge of 4 feet, 8 1/2 inches derives from the original specification for an Imperial Roman army war chariot.

Specifications and Bureaucracies Live Forever.
So, the next time you are handed a specification and wonder what horse's ass came up with it, you may be exactly right. Because the Imperial Roman chariots were made to be just wide enough to accommodate the back-ends of two war-horses.

And the Space Shuttle?
Plus, there's an interesting extension of the story about railway gauge and horses' behinds. When we see a Space Shuttle sitting on the launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These are the solid rocket boosters, or SRBs. The SRBs are made by Thiokol at a factory in Utah. The engineers who designed the SRBs might have preferred to make them a bit fatter, but the SRBs had to be shipped by train from the factory to the launch site. The railway from the factory runs through a tunnel in the mountains. The SRBs had to fit through that tunnel. The tunnel is slightly wider than a railway track, and the railway track is about as wide as two horses' behinds.

So a major design feature of what is arguably the world's most advanced transportation system was originally determined by the width of a horse's ass.