There were several small branches/spurs/lumber RR's over the years between Chama & Lumberton. RG&SW was one of the later ones.
In general, I'd suggest a look at "Rio Grande: to the Pacific" by LeMessena, it has a year-by-year listing of lines built and/or removed, sectional maps to give you an idea of what was where (very vague though) and lots of other information of interest.
To give you a couple of off-beat suggestions:
-The Tennessee Pass line, pre-1886-87, when it was the Red Cliff Branch (+ a bit more)
-Calumet Branch
-Grape Creek Branch
-Iron Mine Branch (Mears Jct to Iron Mine via Poncha Pass & Villa Grove, later the 1st part of the Alamosa Branch(Valley Line) & the Orient Branch.
-Aberdeen Quarry Branch
How big do you consider small? And have you considered, if the program will let you, building
part of a line and then expanding it from either end once you're happy with that part? (for example you might do the 15 miles from La Veta to La Veta Pass and then go onward form there. Heck all the way to Silverton & Denver if your system allows! Keep going and you could do the entire D&RG/D&RGW system for some target year! Then get to work on the DSP&P, the UN and...)(Ok, I get carried away sometimes.
)
Hank