The C&S operated a 3 rail service from Denver to Burns Jct just West of Broomfied where it split from the main line and followed the Marshal Branch to Boulder. The Marshal Branch served the coal fields at Marshal and Superior, and entered Boulder through the CU campus, downgrade near the site of what became Boulder High School football field, crossed Broadway, and curved East through the yard on Water St (now Canyon Blvd) to the Depot at 14th st.
Interestingly, the entire Marshal Branch and the main line from Burns Jct to Denver was electrified, and was also the route of the "Interurban" between Boulder and Denver.
Refer to "The Colorado Road", by Hol Wagner, and, if you can find it, John B Schoolland's "Boulder Then and Now" for photos and more information on this interesting branchline.
I'm sure there must be many photos on DPL.
There is a pedestrian bridge that crosses Boulder Creek West of Broadway that is the location the railroad crossed as it curved back toward the yard and depot. You can also still see the grade where Burns Jct was from the turnpike and at various places from Superior to Marshall.
I used to live in Lyons, and tried to research the narrow gauge early days, I thought and still think it was a fascinating subject, more questions than answers. If memory serves me right there was an article on it referring to a study in the early 1960's which I could not find.
Been a few years but remember seeing what looked like Railroad grade heading north from Boulder in the Lyons direction but not sure anything was ever laid.
Just to add a bit of trivia, When I was young, growing up at MP 14 (Broomfield) on the C&S Main Line, there were many "Fence Posts" around Broomfield made of used ties. Many were three rail ties as you could see the rail spacing from the spike holes.
One time Todd or somebody else posted a picture of some unused grade that was supposed to have been the extension of the GSL&P.I believe it was on NGDF,but it may have been here.In photos of Sunset in the CC 10 era there is a short piece of grade in the background.Also there is a piece of unbuilt grade scar photo in Switzerland Trail.My intention of starting this post was to try and find out if any photos of C&S NG trains or engines running on the 3rd rail line.
I don't believe DB&W freight was interchanged to the C&S and transported to Denver in narrow gauge cars, but it would be interesting to find more about that. For sure there were excursions that were run from Union Station in Denver over the DB&W. I know of at least one photo of a C&S engine on such an excursion but I can't remember if it was in Boulder or up on the line somewhere. DB&W Locomotives were overhauled at the C&S shops, so equipment was at least occasionally taken over the line one way or another.
As I recall, Sam Speas Sr and his locomotive were operated on the old C&N in pre C&S days. That appears in "Goin Railroading". I'll look for that, you've got my curiosity going.....
There is reference to the "Boulder, Left Hand & Middle Park Railroad and Telegraph Co." in Crossen's Switzerland Trail. It was graded some distance up Left Hand Canon and was projected to go over Buchanan Pass into Middle Park.