Engineering

American railroad tracks are 56.5" wide (the "gauge") because the
English built the first railroads in America and they used that width.
Why did they use that width? 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 were used for
building wagons which used that wheel spacing.

Why did wagons have that particular odd wheel spacing? Because older
wagon ruts throughout England used that spacing, and if they changed it,
wagon wheels would break by either falling into or being forced out of
the old ruts, which were 56.5" wide.

The old ruts were that size because the roads were built by the Romans,
who arrived in England in 54 BC and left about 400 AD. Their wagons, and
their chariots before their wagons, used that spacing, and that spacing
was used all over Europe and wherever Rome conquered, because their
wagons used the identical wheel base everywhere. So the modern railroad
track width derives from the Roman chariot.

Why was the Roman chariot track width 56.5"? Because that was the width
of a chariot that would equal the width of two "standard" Roman horses.
Thus, wagon and horses would fit through the same narrow street.
Specifications and bureaucracies live forever!

Such curious dimensions continue today. A space shuttle sitting on its
launch pad has two big booster rockets attached to the sides of the main
fuel tank. These are solid rocket boosters, or SRBs, 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 just wide enough to accommodate a
railroad car, and the railroad track is about as wide as two horses'
behinds, (and we now know why) so the booster rockets were made to fit.

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!