The old Siskiyou Line was laid out in the 1870s and is very, very curvy and slow. SP decided to build the entirely new Cascade Line through Klamath Falls in the 1920s rather than try to realign it. It lost all passenger service in the mid-1950s. Right now it is in the hands of a shortline, CORP, and it is largely 10 mph. However, even if the line was upgraded with new rail, large portions of it would still be 25-30 mph because of the alignment. You would have to build a new railroad on much new ROW to get higher speeds.
As to the former SP West Valley line between Red Bluff (Gerber) and Davis, right now it is cut at the north end, but you could reconnect and relay it and have a 79 mph railroad.