Hi Charles,
There is much speculation on the origins of "standard" gauge (4' 8 1/2"), it was introduced by George Stephenson in Britain and was adopted by many other countries. I don't think there was any particularly well though out reason for it (after all, Richard Trevithick who built the worlds first steam locomotive in 1804 used a 3' tramway and in my home country of Wales non the less!), and Isambard Kingdom Brunel (one of the most incredible Victorian engineers) was a proponent of broad gauge (7' wide))
So at least from what I know there isn't a well thought out reason for 4' 8 1/2"
Graham