I am just about to finish editing our video "Rio Grande's 3rd Division", which is mostly on the Monarch Branch. There is some stuff online, but a good source for Monarch is Trails Along the Columbine-The Monarch Branch.
It really breaks down everything from history, to operations, conversion to NG, and the Barrel Transfer Unit that took the contents from NG to SG.
We also have some of that in the video. It was a really cool line with the steepest grades on the D&RGW at 4.5% and a switchback. Wish I could have seen it in person, but it was a bit before my time.
Greg Scholl