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?

Imperial Rome built the first long distance roads in Europe (and Britain)
for their legions. The roads have been used ever since. And the ruts in
the roads? Roman war chariots formed the initial ruts, which everyone else
had to match for fear of destroying their wagon wheels. Since the chariots
were made for Imperial Rome, they were all alike in the matter of wheel
spacing.

The United States standard railroad gauge of 4 feet, 8.5 inches is derived
from the original specifications for an Imperial Roman war chariot. 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 ends of two war
horses.

Now the twist to the story...

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

When we see a Space Shuttle sitting on its launch pad, there are two big
booster rockets attached to the sides of the main fuel tank. These are
solid rocket boosters, or SRBs. The SRBs are made by Thiokol at their
factory at 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 happens to
run through a tunnel in the mountains. The SRBs had to fit through that
tunnel. The tunnel is slightly wider than the railroad track, and the
railroad track is about as wide as two horses' behinds. So, a major Space
Shuttle 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.

If I remember my 19th Century US History class, pre-Civil War northern lines were standard gauge (4ft, 8.5 inches) while southern railroads commonly used something around 5ft (with some minor narrow-gauge railroads as exceptions both North and South, of course). When it came time to authorize a transcontinental railway by law, Abe Lincoln, being an old railroad lawyer for the Illinois Central (which had a lot of trackage in the South), had pushed for a southerly route and 5ft gauge. But the southern rebellion broke out before the bill passed, and the Pacific Railway Act of 1862 picked a northerly route and standard gauge.

In America, the Erie was of course built to six-foot gauge. The tramway on which the famous "Stourbridge Lion" operated (at Honesdale, PA) was built to a 4' 4" gauge.

"Standard gauge" became standard for the same reason Microsoft's operating system did: not because it was "best", but because it was adopted by the most people.

The story about the solid rocket boosters is true only insofar as every railroad has a "loading gauge" as well as a track gauge. This governs the maximum dimensions of rolling stock. The Association of American Railroads publishes a series of templates (basically cross sections of railroad cars) labeled as things like "Plate B", "Plate C", "Plate H", each incorporating a specific set of dimensions. Virtually all railroads can now accommodate "Plate C" and major routes are cleared for doublestacks ("Plate H"), which need 22 feet above top of rail.