Auther unknowen.

Does the expression, "We've always done it that way" ring
any bells?

The US standard railroad gauge (distance between the rails) is
4 feet, 8.5inches. 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 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? 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 for fear of
destroying their wagon wheels. Since the chariots were made
for Imperial Rome, they were all alike in the matter of wheel

The United States standard railroad gauge of 4 feet, 8.5inches
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 warhorses.
I'm really getting into this origin's great!

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 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 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 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.... and you thought being a HORSE'S ASS
wasn't important!