(Crikey, I come on here to get away from work! *grin*)

Apparently Stephenson later admitted that he would've liked a 5ft gauge, but by then 4ft8.5 was the default, because that was the width of the Stockton and Darlington railway - the idea being that it was open to anyone who wanted to run wagons on it, and the majority of wagons in the area were of this size.

Brunel was a bloodyminded, stubborn git going totally against the grain as usual - the main benefit of his broad gauge railway was economical (as carriage design and track construction eliminated many of the problems). I would've liked to try it out though!