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, and the U.S. railroads
were built by English expatriates. Why did the English build them that
way? Because the first railway lines were built by the same people who
built the pre-railroad tramways, and that's the gauge they used.

Why did the tramway builders use that gauge? Because the people who built
the tramways used the same jigs and tooling that were used to build
wagons, which used that wheel spacing. So why did wagons have that odd
spacing? If they had used any other spacing, the wagon wheels would break
on some of the old long-distance roads in Britain at the time, because
that was the spacing between the wheel ruts.

So who built those old rutted roads? The first long distance roads in
Britain (and Europe) were built by Imperial Rome for their legions. The
ruts in the roads, which everyone subsequently had to match for fear
destroying their wagon wheels, were first formed by Roman war chariots.
Since the chariots were made by and for Imperial Rome, they were all alike
in respect to 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
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: 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. Now, a twist to the
story.

When we see the Space Shuttle sitting on the launch pad, we see two
booster rockets attached to each side of the main fuel tank. These are
solid rocket boosters, or SRBs for short. The SRBs are made by Thiokol at
their factory located in Utah. The engineers who designed the boosters
would have preferred to make them a bit fatter, but the SRBs must be
shipped by train from the factory to the launch site in Florida. The
railroad line from the factory to the launch site runs through a tunnel.
The tunnel is slightly wider than the railroad track, and the railroad
track is about as wide as two horse's rumps.

SO, a 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!

Don't you just love engineering?