The real story is more complicated. While about 4'8" was common, everybody had a different exact size of trackage, and when railroads started to meet and merge it became a squabble.
In the US, the Civil War showed the merit of a national gauge, and Lincoln apparently picked the 4'8.5" gauge as being most-common to the northern railroads.
US railways were about that size because they expected to use UK engines, which were about that size.
But every UK railway was a little different, and Brunel was using 7' gauge. A government committee tested all the gauges, Brunel's 7' was the clear winner, which made everybody unhappy except Brunel, so of course the dolts standardized on the oddest number within the "regular" gauges, 4'8.5". (But a narrower gauge in Ireland.)
I have no idea how Lincoln and UK RR Committee came up with the same odd number. I wonder if all the public history is wrong, and some quiet guy from a RR equipment company jigged for 4'8.5" was passing out money to get his company's gauge made Standard.
The ruts at Pompey (Roman town buried alive in ash) are probably closer to 4'9", and not all uniform.
There is also a theory that 4'8.5" is one-quarter the width of the Queen's Chamber in the Great Pyramid.
http://www.trains.com/Content/Dynamic/Articles/000/000/003/011gsqfq.asp
http://infobluemountains.net.au/rail/horse-ass.htm
http://www.world-mysteries.com/gw_rellis4.htm
Smaller gauges have uses, especially outside wide-area systems. Mine and logging railways are often built narrower. There was a fad for lower-cost regional systems on (various) narrow gauges, and such oddballs persisted in Colorado, Australia, and South Africa/Rhodesia.