The US standard railroad gauge (distance between the 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 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? 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?

Roman war chariots first made the initial ruts, which everyone else had
to match for fear of destroying their wagon wheels and wagons.
Since the chariots were made for, or by Imperial Rome, they were
all alike in the matter of wheel spacing.

Thus, we have the answer to the original question.

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 which horse's rear 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.

And 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.
Thiokol makes the SRBs 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 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 by the
width of a Horse's ass!