The US standard railroad gauge (distance between the rails) is 4', 8.5". That's an exceedingly odd number. Why was that gauge used? Because that's the way they built them in England, and English expatriates built the US Railroads. Why did the English build them like that? 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 then? Because the people who built the tram-ways used the same jigs and tools that they used for building wagons, which used that wheel spacing.
Okay! But why did the wagons have that particular odd wheel spacing? Well, 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's the spacing of the wheel ruts. So who built those old rutted roads? Imperial Rome built the first long-distance roads in Europe (and England) for their legions. The roads have been used ever since.
And the ruts in the roads...? Roman war chariots formed the initial ruts, which everyone else had to match or fear of destroying their wagon wheels. Since the chariots were made for Imperial Rome, they were all alike in the matter of wheel spacing. The United States standard railroad gauge of 4 feet, 8.5 inches is derived from the original specifications for an Imperial Roman war chariot. 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 war chariots were made just wide enough to accommodate the back ends of two war horses. Now the 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 are solid rocket boosters, or SRBs. Thiokol at their factory at Utah makes the SRBs. The engineers who designed the SRBs would 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 railroad line from the factory happens to run through a tunnel in the mountains. The SRBs 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.
NOTE: Axel Vanhooren (Axel.Vanhooren@Village.Uunet.Be) commented on September 15 that:
It appears that the UK did tests in the beginning of the railway area by running on hundreds of kilometers of rails of different distances. They came to the conclusion that the actual distance is what is the best to have the wagons to shake the less possible (maybe there were some more criteria).