- 8 feet 4 1/2 inches is the standardised (internal) railway gauge in the USA
- this is because the early railway tracks in the US were built and designed (at least in a substantial part) by British ex-pats, and, because that was the gauge the trains ran on
- because that was the railway gauge in Britain where the early trains were built.
- The trains in Britain were built to run on track this guage because railway lines followed old trolley cart/tram rail lines
- The tram lines were this gauge because they were built along pathways horse darawn carriages and carts went along
- The carriages and carts were the same measurement (from wheel to wheel) bcause they ran along ruts used by old wagons
- The medieval wagons wore ruts this size because they were standardised so they could run along the old roads between main towns
- Those old roads were mostly old Roman roads still in use
- The Roman roads were standardised to that size to accomodate Roman war chariots
- Roman chariots were standardised to this size because....
- that was the size of two (Roman?) horses's rear ends!
The story spread virally around the net, and s still readily in circulation, although I have not come across who first wrote it. It is often taken prima facie as fact. There are also a number of "counter articles" out there attacking the story, although the critiques vary a lot, and are sometimes, well simply not very interesting, effective and in the worst cases - not factual either.
Contrary to the story, it is true that railway gauge, not to mention carriage, cart and wagon widths have varied over time. Another obvious critiscism is that Roman chariots were not used for warfare, and surely if anything more carts carrying upplies, pack animals and infantry would have been on those Roman roads in Britain, with few if any Roman chariots, and certainly not war chariots. Horses too vary in size and larger and larger horse breeds than the Roman had have been developed through the Middle Ages and into the early Modern draft horse breeds, which are considerably larger than the more pony-like breeds from antiquity.
The NASA Technical Standards Program has produced a good critique in response to the widespread story, here: standards.nasa.gov/documents/RomanChariots.pdf
'I have heard tell of certain wheel ruts having a gauge of you-know-what at the gate to an old Roman fort called Housesteads on Hadrian's Wall in the north of England. Legend has it that George Stephenson based the gauge he used for his locomotives on the width of these ruts. Here's what Housesteads by James Crow (Batsford, London, 1995, pp. 33-34) has to say on the subject:
"The gauge between the ruts is very similar to that adopted by George Stephenson for the Stockton to Darlington railway in 1837 and a 'Wall myth' developed that he took this gauge from the newly excavated east gate. There is a common link, but it is more prosaic and the 'coincidence' is explained by the fact that the dimension common to both was that of a cart axle pulled by two horses in harness (about 1.4m or 4ft 8in). This determined both the Roman gauge and Stephenson's, which derived from the horsedrawn wagon ways of south Northumberland and County Durham coalfields."