The Surrey Iron railway was around 4 foot 2 inches. But this is from an American article that answers the original question.

One-track minds:
The US standard railroad gauge is 4ft 8 ½ in - an exceedingly odd number. Why was it used? Because that's the way they built them in England, and the US railroads were built by English expatriates. The English built them that way because the first railway lines were built by the same people who built the pre-railway tramways, and that's the gauge they used. Why did they use that gauge? Because the people who built the tramways used the same jigs that they used for building wagons. So why did the wagons have that peculiar spacing? Well, if they tried to use any other, their wheels would break on many of the old, long-distance roads in England, because that was the spacing of the wheel ruts.

The first long-distance roads in Europe were built by Imperial Rome. The ruts in the roads, which everyone had to match for fear of destroying their wagon wheels, were first formed by Roman war chariots. The Imperial Roman war chariots were made just wide enough to accommodate the back ends of two war horses. Thus we have the answer to the original question. The US standard railroad gauge of 4ft 8 ½ in derives from the original specification for an Imperial Roman war chariot.