You make some very valid points, but I would like to offer a countering viewpoint. During the height of the narrow gauge era 100 years ago, narrow gauge railroads were quite common, and some fairly extensive. For example, one could travel from Toledo, OH to St. Louis by narrow gauge. And the average citizen wouldn't walk 10 feet to ride the narrow gauge.
But now today were are getting farther and farther away from not only that era, but all of the other "eras" you mentioned. Therefore, as time goes on the uniqueness of narrow gauge railroading in particular is increasing, thereby increasing the historical and curiosity value.
I have casually observed that there seems to be a new tourist operation being launched about every other week. Many of them won't make it of course, but it is an indicator of increased interest in thing railroad.
I fully agree with Boulware's comments on the Trinidad, CO area. It's not well known, but the whole town is a national historic district, and oozes history from every pore. It definitely has potential.
I have driven up the valley from Trinidad, and it is indeed a solid piece of railroad. I'm not familiar with the mine that was mentioned.
Best of luck to John Reed.