There's some very interesting stuff about chariot wheels and railroad gauge here:
particularly the post by Mr. A. W. Worth:
"Chariots were pretty well technologically obsolete by the time of the Romans, because they had developed horses that were big enough to ride. If you go back 1000 years before that, though, to the late bronze age, the time of Troy, the horses were too small to take the weight of a man in armour, and chariots were the ultimate weapon of warfare."
Worth also notes that it's not the Romans who came up with this width, but the earlier Persians.