Of the Definition of Standards...
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 the US railroads were built by English expatriates.
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? The first long distance roads in Europe were built by Imperial Rome for the benefit of their legions. The roads have been used ever since. And the ruts? The initial ruts, which everyone else had to match for fear of destroying their wagons, were first made by Roman war chariots. 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 questions. The United States standard railroad gauge of 4 feet, 8.5 inches derives from the original specification (Military Spec) for an Imperial Roman army war chariot. MilSpecs and Bureaucracies live forever.
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 chariots were made to be just wide enough to accommodate the back-ends of two war horses.
Yet this standard may be even older. 4'8.5" in English measure is 5 pes in Roman measure; the pes "foot" of 12 unciae "inches" was only about 11.3 inches in the later calibration. The reason that the English foot is longer than the Roman is that copies of copies of the standard measuring stick had become discrepant, so England restandardized by the royal appendage of King Edward I "Longshanks" (who apparently had some real clodhoppers at the end of his long shanks).
So the specification was based on a simple integer approximation, 2 equine rears = 5 human feet. Now, the two-horse war chariot was already a very obsolete style by Imperial times. Julius Caesar was amazed to find the Britons still fighting in these things (which they called "essed", a favorite word for some crossword-puzzle constructors). Both the Romans and the Britons had originally learned this style of war from Trojan War refugees: the losing side fled west to Italy (as romanticized in the Aeneid), Spain, and even out the Gibraltar straits to the British Isles (the Danaans, from Argolis in Greece but on the "wrong" side of the war, are well documented in Ireland; and Britain is supposedly named for a Trojan leader). Thus, it is possible that the standard chariot with 5-foot wheel spacing goes back to Troy.
We are of course beyond the reach of reliable records, but it may well be that US Standard Gauge is backward compatible all the way to Trojan Milspec.