It's worth pointing out that this factoid is completely false.

Let me give the actual answers to the questions:

> That's an exceedingly odd number [4'8½" track gauge]. Why was that gauge used?

Various railroad companies in the US over the 19th century moved to standardize gauges as the interconnected nature of railways meant that it was far more beneficial to have a uniform gauge throughout the system rather than dealing with constant gauge switches. The single biggest gauge conversion was the decision of railroads in the South to move to the same gauge as the Pennsylvania Railroad, 4'9", in 1886, converting some 11,000 miles of 5' gauge track to 4'9" in a weekend.

Yes, I said 4'9", not 4'8½". The difference between these two gauges is generally within tolerance, but higher speed traffic generally requires tighter attention to tolerances, and so setting the median at a world uniform 4'8½" standard gauge makes sense. Note that the British legislated the 4'8½" as that country's standard gauge in 1846.

> I see, but why did the English build them like that?

Well, there was a wide variety of gauges in England even after the passage of the Gauge Act, to say nothing of the time when the US was relying on English engineers to build railroads (which is the 1820s; by the mid-1830s, the US already has a pretty thriving homegrown rail industry). There's a broad cluster of track gauges around 5', and that's a pretty natural fit for a rail gauge.

> Well, why did they use that gauge in England?

Stephenson (who built the first practical steam locomotive) chose to use 4'8" for his early railways to match the existing (non-steam) railways then in use, and later expanded it to 4'8½" for the Liverpool & Manchester Railway for better curve engineering. Given the fact that Stephenson was one of the preeminent engineers in England at the time, there was a tendency to use his decisions as a basis for future railway design. However it should be noted that another great titan of English engineering, Isambard Kingdom Brunel, chose a 7' gauge for the Great Western Railway, and this prompted the gauge war that led to the passage of the Gauge Act.

> Okay! Why did their wagons use that odd wheel spacing?

At this point I have to seriously wonder... has this author ever seen a pre-industrial wagon? Because wagons don't have fixed wheel spacing! And they're not around 4'8½"; in fact, they're generally much larger. Look at a railroad car, and notice that it is basically a platform hoisted above the wheels. But wagons are generally built so that the platform is situated between the wheels, with relatively little clearance between the axle and the base of the platform.

> So who built these old rutted roads?

Not worth answering this question since we've deviated from any relevance whatsoever.

> And the ruts?

Going to comment on the answer here: "Roman war chariots." Dude, Romans didn't have war chariots; they became obsolete centuries before Rome existed.

> So, just what does this have to do with the exploration of space?

Absolutely nothing.

You see, the entire discussion I gave so far was on track gauge. But track gauge has no relevance whatsoever to the question of what the maximum size of a load you can carry on track is... that's loading gauge. And whereas track gauge is largely standardized across the world--4'8½"--the loading gauge is very much not. One should note that England, the birthplace of the railroad, generally suffers from extremely tiny loading gauges; the US is among the roomiest loading gauges in the world, especially Plate H double-stacked container traffic.

The main tunnel east of Ohio that's being referred to is likely Moffat Tunnel, which (per Wikipedia) has a width of 18'. Standard US loading gauge is 10'6" wide; the European standards are generally 10'4"; the UK can sometimes go down to 9' loading gauge. The Space Shuttle SRBs are, per Wikipedia, 12'... above all of them. So even the standard loading gauge in the US didn't limit the diameter of the SRB, much less any discussion of the track gauge!