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 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 have that particular odd wheel 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 initial ruts, which everyone else 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 United States standard railroad gauge of 4 feet, 8.5 inches 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 patute came up with it, you may be exactly right, because the Imperial Roman war chariots were made just wide enough to accommodate the rear ends of two war horses.

Thus, we have the answer to the original question.

Now the twist to the story.......

There's an interesting extension to the story about railroad gauges and horses' behinds.