The U.S. 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 people 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 use that odd wheel spacing?

- Well, if they tried to use any other spacing the wagons would break on
some of the old, long distance roads, because that's the spacing of the
old wheel ruts.

So who built these old rutted roads?

- Imperial Rome for the benefit of their legions built the first long
distance roads in Europe. 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 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 army war chariot. Specs and
Bureaucracies live forever. So, the next time you are handed a
specification and wonder what horses ass came up with it, you may be
exactly right. Because the Imperial Roman chariots were made to be just
wide enough to accommodate the back ends of two warhorses.

Now the twist to the story...

There's an interesting extension of the story about railroad gauge and
horses' behinds. When we see a Space Shuttle sitting on the launch pad,
there are two big booster rockets attached to the sides of the main fuel
tank. These are the solid rocket boosters, or SRBs. Thiokol makes the SRBs
at a 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 to the factory runs through a tunnel in
the mountains. The SRBs had to fit through that tunnel. The tunnel is
slightly wider than a railroad track, and the railroad track is about as
wide as two horses' behinds.

So a major design parameter of what is arguably the world's most advanced
transportation system was determined by the ancient Romans, based on the
width of a horse's ass.