In the process of tracking down more details about the history of the RMRRC, I contacted them via their website. A member responded and recommended that I get a copy of Journeys to Yesteryear by David C. Goss that chronicles the group's activities from 1938 to 2003. It is a delightful book especially as it describes every excursion from the group's start to 2003 when the book was written.
A curious C&S-related detail popped up in the very first page of the major Club events. Quoting from the March 1938 issue of Railroad Magazine Carl Hewett wrote "I want to hear from persons interested in forming a rail fan group in Colorado, especially around Denver. Also, I'd like to organize a narrow gauge fan trip over the famous Georgetown Loop."
On March 30th of that year, the Club had its first meeting "in the basement of the Union Pacific Freight House with approximately 20 charter members present."
In the course of research for the book the author came upon a short letter with no date, but likely 1938, and possibly written by Carl Hewett, when he mentions two trips the group planned to take that year. The first was to Cheyenne where they hoped to visit the Union Pacific facilities, but this trip never occurred. The second planned trip was to the Georgetown Loop.
The C&S was already in the midst of abandonment procedures for the line from Idaho Springs to Silver Plume at the time. The ICC had given permission to junk the line starting May 30, 1937, but due to upheaval from those protesting the C&S plan, the case reopened and the date was pushed back to January 31, 1938. More back and forth went on and the date was moved to December 31, 1938. Then, the date was again changed and all was to be revisited on January 31, 1939. This final date secured permission to abandon and the track was taken up a few months later.
Sources:Mac Poor's Denver, South Park & Pacific, 1976 Memorial Edition pg. 404
Daniel W. Edward's A Documentary History of the South Park Line: Vol. 8, pp. 250-252
David C. Goss' Journeys to Yesteryear, 2005, p. 11