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Design Principles
(author unknown)

The U.S. standard railroad gauge -- the distance between the rails -- is 4 feet 8.5 inches. 

That's an exceedingly odd number.  Why was that gauge used?

Because that's the way they built them in England, and U.S. railroads were built by English expatriates.

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.

But why did they use that gauge?

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 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? 

The first long distance roads in England were built by Imperial Rome for their legions.  The roads have been used ever since.

And the ruts? 

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

Thus we have the answer to the original question.  The U.S. standard railroad gauge of 4 feet 8.5 inches derives from the original specification for an Imperial Roman 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.  The Imperial Roman war chariots were made just wide enough to accommodate the rear ends of two war horses.

Now here's a modern twist to the story.  When we see a Space Shuttle sitting on its launch pad, there are two big booster rockets attached to the sides of the main fuel tank.  Those are solid rocket boosters, or SRBs.  The SRBs are made by Thiokol at their factory at 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 railroad line from the factory had 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 is about as wide as two horses behinds.

So, the major design feature of what is arguably the world's most advanced transportation system was based on the width of a horse's ass.