I know you won't see this but I'm going to reply for everyone else. My opinion I've developed through the years is that it comes down to physics and economics on several points.
1. For fixed axle wheels as the gauge gets broader the axle get heavier. They have to be heavier and heavier just to support themselves. Too broad and the train is spending much energy just moving axles. Not to mention the cost of the extra materials to make therm
2. For fixed axle wheels as the gauge gets broader the greater the difference on curves of inside and outside rail. That means the wheels must "slip" more to make a solid axle go around the curve.
3. So as the gauge gets wider, to be efficient, eventually one would have to make independent wheels on each side of the car. Independent suspension creates a whole new set of technological challenges and means the railcar itself has to be designed to withstand horizontal stresses of curves, turnouts, crossings etc.
4. As the gauge gets broader the loads can be bigger and heavier as they will fit on the larger cars, but this demands heavier rails, wider tunnels, wider bridges, more grading, etc. This demands heavier sleepers as they must be longer for the broader gauge. Once again more cost to carry the larger loads.
The physical and economic trade off of all these things dictates a most efficient and economic gauge to be somewhere between six and three feet.
Exactly why 4' 8 1/2 inches instead of something nice and even like 5' I cannot answer. I do not feel like doing all the calculations to find where the theoretical ideal would be. I'm guessing it is close. Plus with today's materials (steel vs iron etc.) it would be a different answer than it was back when the standard was set.