I generally agree with Cecil on this – people doing similar tasks (hauling human-sized cargo on wheeled vehicles) are likely to come up with similar sizes – and this anecdote makes a big deal about this obvious fact.
But I have another fact to add to this.
The “Roman war-horses” would have looked like ponies alongside the English draft horses used to pull wagons. The breeding of horses improved greatly during the intervening 1500-2000 years, and English Draft breeds are quite literally twice the size of Roman horses.
So saying “the railroad gauge is set to match the width of 2 horses’ asses” only makes sense if the horses are the same size. And they aren’t.
)