Re: Dates relating to gauge of Pacific railroad.
I'm not sure the SVRR is really too foreign an issue to the gauging of the Pacific Railroad.
The CP's decision to build their line directly from Sacramento–rather than connecting at Auburn with the SVRR-SP&N–was not made until after the Pacific RR Act–when the requirement that they build 40 miles on their own dime was made law (and 40 miles in the valley was a lot cheeper than 40 miles in the mountains. The foreign iron wasn't that big an issue–the grade itself was an investment, and the CP regularly used foreign iron for side tracks.)
James McDougall was also a director of the SVRR–back when he was congressman. I did not know about his involvement with the early SP–that is interesting. I've never been able to figure out which way McDougall thought (in the 1850s) the railroad should run–around the north end of the Sierra or the south. His involvement with the SP rather explains his interest in the southern route. San Francisco generally favored the southern route as the city was only well situated to be the natural terminus of a railroad approaching from the south. But the whole SF-SJ and WP scheme was designed to make sure that the SV's line to the north was connected with San Francisco–rather than the locally dreaded Benicia-Marysville National Railroad's route.
BTW, I suspect the entire five-foot gauge issue was the historical accident of the old "Elephant" having been built for the South Side Railroad of Norfolk, Virginia. I suspect the SVRR was built to five foot gauge to take advantage of that one locomotive already in California.