I don’t know who wrote this, but it’s one of those bits of trivia that I find fascinating …

The American standard railroad gauge (width between the two 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 the American railroads were built by English expatriates.

Why did the English build them like that? Because the first rail lines were to the same gauge as the pre-railroad tramways, and that’s the gauge they used.

Why did those people use that gauge then? Because the tramways were built with the same gauge as the spacing between two wagon wheels.

Okay – why did the wagons have that particular wheel spacing? Any other spacing would have caused the wagon wheels to break on some of the old long distance roads in England, because the spacing of the wheel ruts on those roads is 4 feet 8.5 inches.

Who built those old rutted roads? The first long-distance roads in England and Europe were built by Imperial Rome 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.

The United States standard railroad gauge of 4 feet 8.5 inches thus derives from the original specification for an Imperial Roman war chariot. So the next time you are handed a specification and wonder what horse’s ass 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. Thus, we have the answer to the original question.

Now the twist to the story…

There’s an interesting epilogue to this tale. A space shuttle has two 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 in Utah. The SRBs haven’t been made wider because they have to be shipped by train from the factory to the launch site. The railroad line from the factory has to run through a tunnel in the mountains and the SRBs have to fit through that tunnel. The tunnel is slightly wider than the railroad track, and the railroad track is the standard gauge: 4 feet 8.5 inches.

A major feature of what is arguably the world’s most advanced propulsion system was influenced over two thousand years ago by the width of two horses’ behinds.

If the designer of the Imperial Roman war chariot were alive today, would he perhaps be surprised by the consequences of his actions?