Then there was this

Before Shasta Dam was built, the Southern Pacific Railroad (now Union Pacific) had its roadbed along the banks of the Sacramento River. This tunnel was built to temporarily reroute the railroad during dam construction until such time as a new permanent route could be created. After all but the center portion of the dam was complete, and the new railroad route had had been finished, the tunnel was then used to reroute the water from the river while the dam was being completed. When this tunnel was built it was started at each end and they met up in the middle. It was pointed out to us by the tour guide that the entire project was done without the benefit of today's technology. The engineers had to use things like slide rules.