I have been modeling the 3rd Division in HOn3 since I made a visit to Marshall Pass in 1966. The decline of the total line from Salida to Montrose really started in the 1920's with the improvement of highway 50 followed by the completion of the Moffat Tunnel in 1932? The Dotsero cutoff was completed in 1934? and the through passenger ng train was stopped in the summer of 1935 after a through train from Montrose to Denver was started and Montrose did not fight the abandonment. The slide above Cedar Creek was used to eliminate freight service by 1948 but might have been sooner except for WW2.
Coal from Crested Butte was the reason the line from Gunnison remained until the 1950's. CF&I switched to a source of fuel from a closer source and natural gas replace coal for utility and home heating. The line was torn up in 1955.
I hope this helps.