Dave, the photo on page 70 of the Rainbow Route seems to show a stone bridge at that location. I suppose it's possible that a portion of the highway bridge is the original rr bridge. Interesting question, and one I'd not thought of before.
Not railroad related, but just below Chattanooga where the three-lane starts is where the Idarado bus (which I was driving at the time) and about eleven other vehicles were knocked into the bottom the the valley by a snow slide while waiting for the Pond Slide to be cleared. There's an account of it, along with a photo of the bus in the creek in John Marshall's book Living and Dying in Avalanche Country.
Sam Bass