I know where there are a LOT of them but they aren't on the internet. The Shasta Dam Visitors Center has thousands of photos of the dam under construction. In a seperate section are hundreds of photos of the railroad and bridge construction. Only way I know of seeing them is to go to the visitors center. I'm not sure that the center is open this time of year.
The Redding trestle was first completed in 1940. At that time they were erecting the bridges on the new alignment around Shasta Dam from both north and south. The Lakehead, Salt Creek, and O'Brien trestles were completed in sequence from the north. The Redding and Pit were to have been completed from the south. After the Redding trestle was first completed there was a flood on the Sacramento in the late fall of 1940 and washed out the concrete footings that the trestle rested on at the south end. Several towers had to be taken down, new footings put in, and the towers reerected. This delayed getting steel to the Pit River bridge long enough that construction was started on the north end. Then work was at both ends. The Pit was completed in the middle rather than working from one end only like the other bridges.
The Shasta Historical Society has a web site that has a few pictures of it. Not really railfan photos though.
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www.shastahistorical.org]
Root around here for bridges, trestles, railroad, Redding.............
jdb