TRC
Much the same thing. I didn't mean that the width was that of 2 horses bums, but it is determined in relation to two horses bums. Roman chariots - as I understood it - were the start. Hence 2 horses with a single shaft between them. The chariots create wheel ruts, and all carts were therefore built to the same dimension because not running in the ruts would likely break your wheel. Then carriage makers were the first to create railway carriages and they simply put the wheels in the same place.
Anyway, its so nice an idea that I want to believe it is true. I think I saw it on a program years ago, which (again, I think) was called "Connections" presented by James Burke.
(you spell it your way, I'll spell it wrongly)