Two things:
First, there is a series of books to which I cannot remember the name of the author or publisher. One is titled "Chama to Cumbres with a little Chili," there is one on Alamosa, Durango and a fourth on Gunnison. I am sure someone here knows more info on these; my copies are from my fathers collection and at my parents house. Inside the Gunnison book I believe is a pretty decent drawing of a C-21 though it has been years since I looked.
Second, The Narrow Gauge and Shortline Gazette (NG&SL) and Benchmark Publications have digitized all of their issues from the begining until the end of 2014. These come on a CD set that can be searched by title, author, description or even a single word. The CD is going for $140, which seems a little steep, but figure over 300 magazines and you are pennies for each.
We purchased one of the CD sets and I have browsed it a tad. All pages, including plans, pictures, articles and ads can be saved to a .pdf and are excellent quality!
Casey Akin