The U.S. standard railroad gauge (distance between rails) is 4 feet 8.5 inches. That is an exceptionally odd number. Why was that gauge used?
Because that’s the way they built them in England, and the U.S. Railroads were built by English expatriates.
Why did the English build them that way? Because the first rail lines were built by the same people who built pre-railroad tramways, and that’s the gauge they used.
Why did "they" use that gauge then? Because people who built the tramways used the same jigs and tools that they used for building wagons, which used that wheel spacing.
So why did the wagons have that particular odd spacing? Well 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 who built those old rutted roads? The first long distance roads in Europe (and England) were built by Imperial Rome for their legions. The roads have been used ever since.
And the ruts in the roads? The ruts in the roads, which everyone had to match for fear of destroying their wagon wheels, were first formed by Roman war chariots. Since the chariots were made for (or by) Imperial Rome, they were all alike in the matter of wheel spacing.
The U.S. standard railroad gauge of 4 feet 8.5 inches derives from the original specification for an Imperial Roman war chariot.
Specifications 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 war chariots were made just wide enough to accommodate the back end of two warhorses. Thus, we have the answer to the original question.