The distance between the rails of U.S. standard gauge railroads is 4 feet 8 ½ inches. This distance is a rather strange number. For you metric fans, the equivalent 1.4351 meters is an even stranger number. Where might this number have come from? This spacing was used on some English railroads and some of the first engines in the United States came from England.
Why did British railroad builders use this spacing? Some of the first British rail lines were built by the builders of the tramways and that is the spacing that they used.
So, why did the tramway builders use this spacing? The tramway builders were also wagon builders and they had existing jigs used for building wagons, which had that wheel spacing.
So why did wagons have such a strange wheel spacing? If wagons used any other wheel spacing, the wheels would break on certain of the old, long-distance British roads, which had ruts of that spacing.
So why did these roads have ruts of that spacing? Roman engineers built a network of long-distance roads throughout Europe for both commerce and military use by the Roman legions. Standardized Roman chariots made those ruts initially, and anyone who wanted to use those roads had to conform to that standard or risk destroying their wheels. Some of those roads remained in near continuous use until the 20th century and the advent of paved roads for automobiles and trucks.
So the U.S. standard gauge railroad of 4 feet 8 ½ inches was derived from Imperial Rome’s standard for the war chariot—wide enough to accommodate the rears of two Roman war horses. Standards live forever—something to keep in mind the next time you come across a strange requirement and wonder from where it came. So now we have an answer to the original question.
Many thanks to David Upstone, BSI (retired, the longtime secretary of ISO/TC 121) from whom I first heard this story—a long time ago in pre-Internet days.
It is a wonderful story, but …
Roman roads did have consistent ruts that where spaced a little wider than 4 feet 8 ½ inches. Heavy usage of unimproved roads does create ruts that are wider than the wheel spacing. Just look at the sections of the Oregon or Santa Fe Trails that are still remaining in the American West (I have seen some of these the ruts). You would quickly destroy your wagon if your wheel spacing didn’t match those of your predecessor who had made the ruts.
In both America and England, early railroads had all sorts of track spacing. The earliest railroads were either man powered (often in mines) or pulled by draft animals. In the United States, there were all sorts of narrow-gauge railroads, particularly in the Appalachian Mountains (for logging and mining). During the American Civil War, Southern and California railroads were incompatible (wider spacing) with the railroads of the Northeast (primarily 4 feet 8 ½ inches spacing). The United States did receive an early locomotive from England (designed by George Stephenson, inventor of the steam locomotive) with a 4 feet 8 ½ inches gauge. New England factories were soon making large numbers of compatible locomotives (just a few miles down the road from where I live).
During the Civil War, the U.S. Congress passed a law to develop an east-west transcontinental railroad. Such a railroad needed consistent track spacing so that a train could traverse the whole length. Given the politics of the day, guess which track spacing they chose.
Nonetheless, standards do live forever.
Dave Osborn is senior manager of international standards and regulations with Philips Healthcare.