Worldbuilding--How a horse's rear size dictates how we blast into space

In FORCED MATE, the way my aliens tell time (officially) is a throw back to their low tech ancient days.
"The old names stuck," the alien Xirxex explains to an incredulous human.

It's not so implausible. A correspondent sent me this incredible sequence of events, which turns out not to be true, www.snopes.com.
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Did you ever wonder why the US standard railroad gauge (distance between the rails) is 4 feet, 8.5 inches?

Because that's the way they built them in England, and English expatriates built the US Railroads. The English built 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? Because the people who built the tramways used the same jigs and tools that they used for building wagons, which
used that wheel spacing. And, they used that particular odd wheel spacing because, 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 the gauge of American rails was determined by the width of the ruts in English roads? Who built those old rutted roads?
Imperial Rome built the first long distance roads in Europe (and England) for their legions. The roads have been used ever since. Roman war chariots formed the initial ruts, which everyone else had to match for fear of destroying their wagon wheels. Since the chariots were made for Imperial Rome, they were all alike in the matter of wheel spacing.

The United States standard railroad gauge of 4 feet, 8.5 inches is derived from the original specifications for an
Imperial Roman war chariot. Why was a war chariot that width? Because the Imperial Roman army chariots were made just wide enough to accommodate the back ends of two war horses!

The story doesn't stop there!

When you 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. The SRBs are made by Thiokol at their factory at Utah. The engineers who designed the SRBs would 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 happens 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, as you now know, is about as wide as
two horses' behinds.

So, a major Space Shuttle 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 bottom.

NASA, tell me it isn't so!

Best wishes,
Rowena