American railroad tracks are 56.5" wide (the "gauge") because the English built the first railroads in America and they used that width. Why did they use that width? 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 were used for building wagons which used that wheel spacing.

Why did wagons have that particular odd wheel spacing? Because older wagon ruts throughout England used that spacing, and if they changed it, wagon wheels would break by either falling into or being forced out of the old ruts, which were 56.5" wide.

The old ruts were that size because the roads were built by the Romans, who arrived in England in 54 BC and left about 400 AD. Their wagons, and their chariots before their wagons, used that spacing, and that spacing was used all over Europe and wherever Rome conquered, because their wagons used the identical wheel base everywhere. So the modern railroad track width derives from the Roman chariot.

Why was the Roman chariot track width 56.5"? Because that was the width of a chariot that would equal the width of two "standard" Roman horses. Thus, wagon and horses would fit through the same narrow street. Specifications and bureaucracies live forever!

Such curious dimensions continue today. A space shuttle sitting on its launch pad has two big booster rockets attached to the sides of the main fuel tank. These are solid rocket boosters, or SRBs, made by Thiokol at their 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 from the factory had to run through a tunnel in the mountains. The SRBs had to fit through that tunnel. The tunnel is just wide enough to accomodate a railroad car, and the railroad track is about as wide as two horses' behinds, (and we now know why) so the booster rockets were made to fit.

The 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!

I modified this story from one already existing on the web.
The story has its critics, who point out that America has had as many as 20 railroad gauges in its history, that Rome did not use chariots in war (true - but they did use them in transportation, along with lots of wagons), and that booster rockets are now over 12 feet in diameter. However, critiques which compare what exists now with what existed in a previous era have made no argument, and none of the criticisms give an adequate explanation as to why we should suppose that railroad gauges are Unrelated to previously existing wagon ruts and the vehicles which created them.

The story, as I've woven it, is pretty colorful, and may in fact be true.

True, or interesting enough to make several other websites copy my web page as if it is their own.
Word for word, in many cases. Almost identical, in other cases.
They should feel the shame, but they do not. That is what the Internet has become.

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