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 question.  The United States standard railroad gauge of 4 feet, 8.5 inches derives from the original specification for an Imperial Roman army war chariot.

Specs 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.
 

Now the twist to the story....

There's an interesting extension of the story about railroad gauge and horses' behinds.  When we see a Space Shuttle sitting on the launch pad, there are two big booster rockets attached to the sides of the main fuel tank.  These are the solid rocket boosters, or SRBs.  The SRBs are made by Thiokol at a 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 to the factory runs through a tunnel in the mountains.  The SRBs had to fit through that tunnel.  The tunnel is slightly wider than a railroad track, and the railroad track is about as wide as two horses' behinds.  So a major design feature of what is arguably the world's most advanced transportation system was determined by the width of a horse's ass!