Here is a look into ENGINEERING specs that is very interesting,
educational, historical, completely true, and hysterical all at
the same time:

The U.S. standard railroad gauge (width between the two 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 the 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. Why did "they" use
that gauge 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 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 Europe (and England) were built by
Imperial Rome for their legions. The roads have been used ever since.  
And the ruts in the roads? Roman war chariots first formed the initial 
ruts, which everyone else had to match for fear of destroying their wagon
wheels. Since the chariots were made for (or by) Imperial Rome, they 
were all alike in the matter of wheel spacing. The United States 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, because the Imperial Roman war chariots were 
made just wide enough to accommodate the back ends of two war horses.  
Thus, we have the answer to the original question.

Now the twist to the story... There's an interesting extension to 
the story about railroad gauges and horses' behinds. 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. These are solid rocket
boosters, or S.R.B.'s. The S.R.B.'s are made by Thiokol at their 
factory in Utah. The engineers who designed the S.R.B.'s might have 
preferred to make them a bit fatter, but the S.R.B.'s 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 S.R.B.'s 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' rears. So, the major 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!