> Total crap! Such misinformation has been perpetuated ever since the internet began, and maybe even prior to THAT.

Yes prior to the internet. I'd always heard that 4' 8 1/2" was based the spacing of Roman chariot and wagon wheels.

It turned out this isn't true at all. "Standard gauge" began as Beaumont's wooden rail track of 1630, built for a coal haulage line at Newcastle on Tyne, using wood spoke wheels with double flange iron tires. Gauge was 5' measured over the outsides of the stingers, which were rectangular in cross section. Later on iron plates were added to the tops of the wood stringers to extend their life. By 1789, a man named Jessop (also in the UK) used 100% iron wheels running in angle irons, with the vertical of the angle iron on the inside, what is now called the "gauge side". The horizontal leg of the angle was laid flat on the sleepers, with the outer end of the L set to Beuamnont's gauge, a backwards compatibility thing. In 1820, Birkenow developed the T-head fishbelly rail with chairs, also using Beaumont's gauge for compatibility sake. Since the rail heads were 2 1/4" wide, this made for an inside gauge of 4' 8 1/2". When Stephenson built the Stockton and Darlington in 1825, he used Birkenow's scheme because it was the most advanced at the time. Then when he built the Liverpool and Manchester five years later, he said to "Make them of the same width. Though they may be a long way apart now, depend on it; they will be joined together some day". And as the trite and hackneyed phrase goes, "and the rest is history".

Source: "Safe Railway Working", by C. E. Stretton, 1886.

For a look at those "log rails" and double flange wheels, see: [en.wikipedia.org] which dates from the 16 century, around 100 years before Beaumont, and there are indications that this particular cart was a rather "late model design" of even older ones.