I would be interested to know about the location of some of the 50+ industrial narrow gauges still in operation.
South Louisiana, which is where I live, used to be covered with plantation railroads for the sugar industry. Alas, other than W.E. Butler's book on the subject, there is not much you can find out about these great little railroads.
There was also a narrow gauge logging line in the "boot" called the Kentwood & Eastern. One of its locomotives traveled to the ET&WNC where it ended its days.
I would venture a guess that the reason most people don't show much interest in the existing narrow gauge industrials is that they are powered by diesels.
I made a post a couple of months ago about the White Pass 90 class and the response was "ugh-diesels." I think the 90's are really unique. While similar GE's have been exported to Southeast Asia and South America, there are no other's of their kind in North America.
My guess that part of the reason you see so little about non- Colorado narrow gauges is the utter saturation of Colorado narrow gauge in the railfan book market and modeling market. Just how many books are there about the RGS? How many about the White Pass? How many about the EBT? Argent Lumber Co.? All interesting railroads with really interesting equipment. You just don't have an opportunity to learn as much about the others because it the information is not readily available like it is for the Colorado narrow gauge.