In my email this week I found this intriguing article entitled "Horse's Ass". I quote it here in full:

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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 English expatriates built the US Railroads.
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?
Imperial Rome built the first long distance roads in Europe (and England) 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 army chariots were made just wide enough to accommodate the back ends of two war horses.
Now, the twist to the story.
When you 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 would 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, as you now know, 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.
- And -
You thought being a HORSE'S ASS wasn't important!

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I have tried to identify the original author but like so many other apocryphal stories, the best I could do was to establish it has been posted from one bulletin board to the next, and copied and send by email to thousands of recipients (1). My curiosity was piqued though. Since reading it I have been digging into the truth of the author's assertions.

First, there's that width measurement of 4 feet, 8.5 inches. It turns out that this is the current international standard. Indeed,

"Sixty percent of the world's railways use a gauge of 4 ft 8½ in (1435 mm), which is known as the standard or international gauge. Gauges wider than standard gauge are called broad gauge, those smaller are called narrow gauge" (2).

The author's assertion that "that's the way they built them in England, and English expatriates built the US Railroads", belies a struggle between the two towering giants of Industrial Revolution engineering. In England in the late 1700s and early 1800s, there was competition between three gauges. The standard gauge of 4 ft 8½ in (1435 mm) was chosen for the first main-line railway, the Liverpool and Manchester Railway (L&MR), by the British engineer George Stephenson (3). He chose it because it was the de facto standard for the colliery railways where Stephenson had worked. Whatever the origin of the gauge it seemed to be a satisfactory choice: not too narrow and not too wide. Meanwhile, that other great British engineer of the age, I.K. Brunel, was pioneering the broader gauge of 7 ft 0¼ in (2140 mm) for his Great Western Railway (GWR) "partly because it offered greater stability and capacity at high speed, but also because the Stephenson gauge was not scientifically selected" (4). The Eastern Counties Railway (ECR) was using a third gauge of 5 ft, but it later gave up in favor of Stephenson's chosen measure.

The matter was settled in 1845 when a British Royal Commission recommended adoption of 4 ft 8½ in as standard gauge, and in the following year Parliament passed the Gauge Act, which required that new railways use standard gauge. Since the British built railways throughout their empire, that was the standard set for use worldwide. Nevertheless in Britain, several narrow gauge railway networks still survive even today, including the Great Little Trains of Wales (5).

As far as the English building America's railways goes, "given the nation's recent independence from the United Kingdom, arguments based on British standards had little weight" (2). The result was "some railways, primarily in the northeast, used standard gauge; others used gauges ranging from 4 ft (1219 mm) to 6 ft (1629 mm)". Agreement on the standard gauge was not reached until the 1880s, after the Civil War compelled North and South to agree on common standards to facilitate trade. The true standard gauge of 4 ft 8½ in was agreed.

Then there's that line about "Specifications for an Imperial Roman war chariot". I looked but could not find anything in the literature about a written specification. Raymond Chevallier in his book Roman Roads (1976) describes several different kinds of wheeled vehicles used in Roman times from two wheeled carts (carpentum) to four wheeled wagons (plaustrum) and covered carriages (angaria) used for delivering the mail. There is a beautiful full scale reconstruction of such a covered carriage in the Römisch-Germanisches Museum in Cologne, Germany. The chariot (carus) is more problematic. There are no depictions of two wheeled chariots on the columns of Trajan or Marcus Aurelius in Rome. This suggests chariots were not part of regular transport or equipment of the legions. Elaborately decorated two wheeled chariots seem to have been restricted to use in the great military triumphs while stripped down light weight versions were used for high-speed racing at the circus. 

Chariots drawn by four horses (quadriga) - rather than two (biga) - are frequently shown on commemorative issues of coins from the days of the republic right through the Empire as well as on friezes on the triumphal arches themselves, mostly notably that of Titus celebrating the Jewish War. The number of horses employed in these chariots is significant in the present discussion as this is double or quadruple the number used in collieries centuries later, which is presumed to be the rationale for Stephenson choosing 4 ft 8½ in.

Accepting that the triumphs were uncommon events, Roman road widths give us some clue as to how wide regular roads were, and by inference the axle widths of the vehicles. Augustus' had

"Issued decrees (known through surveyors' writings) fixing the width of a decumanus maximus [the main street in a Roman fort or city] as 40 feet (13m), that of a cardo maximus as 20 and of other decumani and ordines as 12. By-roads were allowed 8 feet" (Chevallier, page 88).

Can anything be deduced about the axle width or gauge of wheeled vehicles? Wheel ruts in ancient Pompeii's city roads and elsewhere, suggest variation from 1.05m to 1.85m. Chevallier proposes

"It is quite possible that careful study of the standards used could yield evidence about dating (1.3m in antiquity, 1.45m in the Middle Ages" (Chevallier, page 89).

Significantly varied enough, then, for me to be able to refute as unproven the precise standard of 4 ft 8½ claimed for Roman chariots: our evidence is just too fragmentary for us to know for certain.

So the conclusion is that the sizing of those SRBs shipping from Utah to Florida by railway probably owes less to Roman chariots and more to the pit ponies who laboured in England's collieries of the 1700s (6). Like a lot of stuff on the Internet, the article has just enough factual information to have the air of truth. Dig deeper and the foundations are shaky. You should not believe everything that you find on the World Wide Web or that lands unsolicited in your mail box. With a little healthy skepticism you can avoid being a horse's ass!

References

Raymond Chevallier, Roman Roads, London 1976

1. 
   
http://www.hard-light.net/forums/index.php/topic,42607.0.html
2. 
   
http://en.wikipedia.org/wiki/Railway_gauge
3. 
   
http://en.wikipedia.org/wiki/George_Stephenson
4. 
   
http://en.wikipedia.org/wiki/Isambard_Kingdom_Brunel
5. 
   
http://www.greatlittletrainsofwales.co.uk/
6. http://ludb.clui.org/ex/i/UT3163/