Here is a look into the corporate mind that is very interesting,
educational, historical, completely true, and hysterical all at the same
time:

The US standard railroad gauge (width between the two 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? Roman war
chariots first formed the initial ruts, which everyone else had to match
for fear of destroying their wagon wheels. 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.
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 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. 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 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
from the factory had 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, the major 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! 






RM adds:  http://www.snopes.com/history/american/gauge.htm documents
that the above is almonst entirely a crock.  Quoting:



Origins:   This is one of those items that -- although wrong in many of
its details -- isn't exactly false in an overall sense and is perhaps
more fairly labelled as "True, but for trivial and unremarkable
reasons." Marvelling that the width of modern roadways is similar to the
width of ancient roadways is sort of like getting excited over a notion
such as "modern clothes sizes are based upon standards developed by
medieval tailors." Well, duh. Despite obvious differences in style,
clothing in the Middle Ages served the same purpose as clothing today
(i.e., to cover, protect, and ornament the human body), and modern human
beings are very close in size to medieval human beings (we are, on
average, a little bit taller and heavier than we were several centuries
ago, but not much), so we naturally expect ancient and modern clothing
to be similar in size.

So, rather than going into excruciating detail about the history of
transportation, we'll simply note that roads are built (or worn) to
accommodate whatever uses them, and that for many centuries prior to the
advent of railroads, what travelled on roads were mostly wheeled
conveyances, pulled by beasts of burden (primarily horses), carrying
passengers and goods. Physical conditions dictated some of the
dimensions of those conveyances (such as the width of their axles) and
largely ensured that they would fall within a fairly narrow range of
variation: Horse-drawn vehicles, whether they were chariots or carts or
carriages, all served similar functions, so practical considerations
(e.g., the speed at which horses could travel, the amount of weight
horses could pull, the number and arrangement of horses that could be
controlled by a single driver) required that they be relatively similar
in size as well.

[Article goes on to explain how/why this same process of adaptation
extends to rail vehicles, explaining how early rail cars were in fact
adapted horse-age carriages. It also pointed out that the reason 
many American railroads used the same gauge as British ones was purchase
by the Americans of British equipment, _but_ that nonetheless three
entirely different gauges came into use in the USA.  Standardising on 
British gauge owed not to slavish devotion to ancient British standards
than to the fact that other gauges were mostly used by the losing side of
the American Civil War.  The article also points out that data from the
US Army's Rail Transport in a Theater of Operations document makes clear 
that no railroad equiment is "only slightly wider" than 4' 8 1/2".]