The standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches. Why?
Because that's the way they built them in England.
Why did the English build them like that?
Because they used the same tools that they used for building wagons, which used that wheel spacing.
Why did the wagons have that particular wheel spacing?
Because that was the spacing of the wheel ruts on the roads in England & if they tried to use any other spacing, the wagon wheels would break.
So who built those old rutted roads?
Imperial Rome built the first long distance roads for their legions.
The roads have been used ever since.
Roman chariots formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels.
Since the chariots were made for Imperial Rome, they were all alike in the matter of wheel spacing. Therefore the standard railroad gauge of 4 feet, 8.5 inches is derived from the original specifications for an Imperial Roman war chariot.
The chariots were made just wide enough to accommodate the rear ends of two horses.
So the next time you are handed a specification/procedure/process and wonder 'What horse's ass came up with it?', you may be exactly right.
Now, a twist to the story :
When you see a Space Shuttle sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank. These SRB's are made at a factory in Utah. The Engineers who designed the SRB's would have preferred to make them bigger, but they had to be shipped by train. The railroad line from the factory runs through a tunnel in the mountains, and the SRB's had to fit through that tunnel. The tunnel is slightly wider than the railroad track, and the railroad track, as you now know, is about as wide as two horses' behinds.
So, a major Space Shuttle design feature, of what is arguably the world's most advanced transportation system, was determined over two thousand years ago by the width of a horse's ass.
Gives one a new perspective of a horse's ass.