The current bridge that I-5 uses is the same bridge as in your photo. The changes that were made was that the upper deck for the bridge was widened to accomodate the extra 2 lanes for when I-5 was built.

The original main line followed what was the Sacramento River before Shasta Dam was built. About a 10 mile portion of the original main line south of the Dam is being used as a walk/bike trail. The original main ran through the once thriving copper town of Kennet which now lies under Shasta Lake. The new main meets back up with the original main at a few miles south of Delta. The original main I believe was surveyed by Samuel S. Montague and not by William Hood who did the survey over the Siskiyou Mountains.

If you want more information on the line relocation around Shasta Lake I recommend getting a copy of John Signor's book "Southern Pacific's Shasta Division." It is a very informative book with plenty of maps including the line relocation.