4 feet and 8 1/2 inches
Railroad track gauges are the same in many parts of the world. Whether you look in the U.S or in Europe, it's either 4 foot and 8 and a half inches or 1435 mm respectively - which is exactly the same. Not a handy figure and you might wonder how come engineers started using exactly this weird figure.
Well, first of all, railroads were first built in Britain, so the foot-and-inch angle is easy to understand. Besides, railroads in the U.S were constructed in the 1840s by British immigrants, so it was logical for them to use the same gauge they knew from the Old World.
But still, why did the British use 4' 8 1/2" ?
Now, because the first tracks in Briatin were built by the same people that built the tramways and horsecars before railroads came along. And they built those tracks using 4' 8 1/2". Because they used the same tools and implements they already had for building coaches and wagons. Coaches that had all a wheel spacing of 4 feet 8 1/2 inches and wagons with wooden spoke wheels whose life span was quite short on the deeply rutted long distance freight roads. The teams had to stay in the ruts formed over the long centuries since the roads had been built, or the wagons risked breakdowns. Ruts that had a 4 feet and 8 1/2 inches gauge.
But who had build the roads? Well, not only in Britain, all over Europe long distance roads had been built by the Roman Empire that needed to move around her legions quickly all over the continent. It seems the first ruts on those roads were made by roman war chariots, whose axle span had to be such that it would line up behind two fat war horses. And that seems to have been 4 feet and 8 1/2 inches all over the Roman Empire.
Voilà l'éxplanation - this explains a lot.
So, if ever you look at a set of specs that doesn't seem right and you ask yourself "What horse's arse has desinged these?" - You'll know where that expression comes from. It's the arses of Roman Emperial horses. And the gauge is still 4 feet and 8 1/2 inches.
Now, coming back from the Roman Empire into todays modern world ... if you look at a space shuttle sitting on its launching pad, you see the two solid rocked boosters on both sides of the main fuel tank. They look oddly narrow on the shuttle, and you might ask yourself, what horse's arse hasn't made them any wider. These RSRBs [Re-usable Solid Rocket Boosters], as they call them are manufactered by Thiokol Propulsion, a company in Brigham City, Utah. The specs had originally indeed called for a wider rocket body. But the things have to be transported to and from the plant by railroad, as they are - even taken apart for transportation - to big to be shipped around on roads. And as Utah is in the Rockies, there's a tunnel on railway line to the Thiokol plant. And the tunnel is only slightly wider than the railroad track. And the railroad track has the gauge of two Empirial Roman War Horse Arses.
So, we've come to realise that some of the specs for a very modern transport system has been laid down more than 2000 years ago by the span of two horse's behinds......