In the big picture, I think that asking for the origin of standard gage is an impossible question to answer. It is like asking for the origin of time.
And, a few years back, there was an article in Trains about Hitler's proposal for a wider gauge--about nine feet, as I recall
As to the additional half inch, could it have been that the actual gauge of the chariot wheels was some particular Roman measure, and after the English measuring system was standardized the gauge was discovered to be 4' 8 1/2"?
Johnny
If you use the search function of this forum and search "gauge" and "chariot" you will find links to several long discussions about this theory we have had over the years.
Basically all of the attempts to analyze the "Roman Chariot" stories do seem to conclude that there is some truth to it in the sense that Stephenson's "standard" gauge did derive from the most common size axle width for wagons of the type used in the industries that first embraced rail transportation (mining) but that this has more to do with the fact that an axle width of approx. five feet (give or take) was necessary to allow sufficient room for a carthorse to fit between the wagon shafts..
"I Often Dream of Trains"-From the Album of the Same Name by Robyn Hitchcock
Having the gage evolve generally from wagon tracks seems entirely plausible. Standard gage became standard because it was generally deemed adequate and the more it was used, the more attractive it became. It really does not require a specific plot to explain the width of standard gage. The dimension of 56 ½” might seem odd and in need of an explanation, but there is nothing odd about 56”. The extra ½” seem a little quirky, but the explanation that it was added for flange clearance seems entirely plausible.
However, this does get into some bewildering issues as to whether you add the ½” to the track gage or deduct it from the wheel gage. To achieve more flange clearance, they could have made the wheel gage 4’-7 ½”. I guess it depended on whether it was more work to change the wheels or the track.
Track gage is objective because it is measured inside the rail head sides. Whereas wheel gage is a measured between arbitrarily established vertical planes in space.
This is an urban legend that has been proven wrong several times over -- check Wikipedia, as an example.
https://en.wikipedia.org/wiki/Standard_gauge
and I believe Snopes has also debunked this theory.
This has passed on the internet many times over the years. And there are many who have debunked the claim, too. So I'm not sure it is 100% factual or true.
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For those of you that are interested in why the track gauge in the US is 4'81/2", I offer the following link to an interesting article: http://www.straightdope.com/columns/read/2538/was-standard-railroad-gauge-48-determined-by-roman-chariot-ruts
"The Straight Dope" started many years ago in Chicago as a sort-of underground publication, and has now become a rather fascinating collection of tidbits of information.
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