The U.S. standard railroad gauge (distance between the rails) is 4
feet, 8.5 inches. That is an exceedingly odd number. Why was that
gauge used? Because that's the way they built them in England, & the
U.S. railroads were built by English expatriates. Why did the English
build them that way? Because the first rail lines were built by the
same people who built the pre-railroad tramways, & that's the gauge
they used. Why did "they" use that gauge then? Because the people who
built the tramways used the same jigs & tools that they used for
building wagons, which used that wheel spacing. So why did the wagons
have that particular odd 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? The ruts in the roads, which
everyone had to match for fear of destroying their wagon wheels,
were first formed by Roman war chariots. Since the chariots were
made for (or by) Imperial Rome, they were all alike in the matter of
wheel spacing. The U.S. standard railroad gauge of 4 feet-8.5 inches
derives from the original specification for an Imperial Roman war chariot.

Specifications & bureaucracies live forever. So the next time you
are handed a specification & 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 end of two war
horses. Thus we have the answer to the original question.

When we see a space shuttle sitting on its launching pad, there are
two booster rockets attached to the side of the main fuel tank. These
are solid rocket boosters, or SRB's. The SRB's are made by Thiokol at
their factory in Utah. The engineers who designed the SRB's might
have preferred to make them a bit fatter, but the SRB's had to be
shipped by train from the factory to the launch site. The railroad
line from the factory had to run thru a tunnel in the mountains. The
tunnel is slightly wider than the railroad track, & 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!!!

Marvelous, Dude. Loved it. Must be why that part of the horse's anatomy shows up so often in people these days.

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Gunslinger
'Never teach a pig to sing. It wastes your time and annoys the pig.'