The US 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 US 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 extra-terrestrial 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. These are solid rocket boosters,
or SRBs. The SRBs are made by Thiokol at their factory
in 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 determined over
two thousand years ago by the width of a horse's ass.
And you wonder why it's so hard to get ahead in this world...