This really is an interesting picture, though most of us have seen it many times before, but this is an excellent scan of Jackson's amazing photographic skills.
The SN bridge indeed is not in this picture, though the track on up Cement Street is there. My conjecture is that the construction of the SN engine house required a better connection than what could be done working through the existing yard trackage--we see this problem today with the new construction on how to access the mainline with the Shennandoah spur clogged with cars and the 493. Back in 1910 or 12, that was an active industrial spur with several businesses getting cars of material posing the same access problem. Also, the course of Cement Creek is now on the north side next to the remaining approach grade as I think there was some channel changes made some years back. The Silverton town ordinances conveniently leave out permission to use Cement Street from the engine house to Cement Creek, though the ordinances cover all the other railroad usage of public streets.
I am intrigued by the reversing loop at Chattanooga--as soon as the snow melts, I plan to scout that out to see what's left. I do know that the ties on the top of Red Mountain Pass are still there, about 2 feet under the blacktop of US 550 as I saw them sticking out several years ago when CDOT was doing some ditch work adjacent to the shoulder of the highway--I guess that's what is called building on the past.