On the E-N end of things it even gets more confusing as John Sexton, the manager of the Eureka-Nevada, was variously involved with dealing with used locomotives and promoting other railroads.
One of these other railroads was a line called the "Surprise Valley." The Surprise Valley is the next valley east, over the mountains, from Alturas; I first became aware of this when I picked up a book about Flanigan, Nevada, during a visit to the Nevada Historical Society. This is the probable source of the freight cars on the E-N that were lettered "S.V."
Brian Norden