Roads: Specifications & bureaucracies live on forever! 

The US standard railroad gauge (space between the rails) is 4 feet 8½ inches. z
That is a very odd number. Why was that gauge used?
Because that's the way they built them in England and the US railroads were built by English expatriates.
Why did the English build them that way? Because the first rail lines were built by the same people who built the pre-railroad tramways, and that's the gauge they used.
Why did they use that gauge? Because the people who built the tramways used the same jigs and tools that were used for building wagons, which used that wheel spacing.
So why did the wagons have that particular spacing? If they tried to use any other spacing, the wagon wheels would break on some of the old, long distance roads in England because that was the spacing of the wheel ruts.
So who built those old rutted roads? The first long distance roads in Europe (including England) were built by Imperial Rome for their legions and they have been in use ever since.
And the ruts in the roads? The ruts, which everyone had to match for fear of breaking their wagon wheels, were first formed by chariots. Since the chariots were made for (or by) Imperial Rome, they were all alike in the manner of wheel spacing.
The US standard railroad gauge of 4 feet 8½ inches was derived from the original specifications for the Imperial Roman war chariot.
Specifications and bureaucracies live on forever!
So, the next time you are handed a specification and you wonder what bureaucratic horse's ass came up with it, you may be exactly right, because the Imperial Roman war chariots were made just wide enough to accommodate the rear ends of two horses.
There we have the answer to the original question but now for a new twist to the story.
When we see the space shuttle sitting on its launching pad, there are two booster rockets attached to the side of the main fuel tank. These are solid fuel boosters that are made by Thiokol at their plant in Utah.
The engineers who designed the boosters would have preferred to make them larger in diameter but they had to be shipped by rail from the factory to the launch site. The railroad from the factory had to pass through several tunnels in the mountains that are, you guessed it, just slightly wider than the railroad track which is about two horse butts wide.
Incredibly, a major design feature of what is currently the world's most advanced and technologically sophisticated transportation system was determined over two thousand years ago by the width of a horse's ass.
Don't you just love engineering?