Indeed, a look at a California Topography map shows that Beckwourth Pass only solves part of the problem.You would still be East of the Sierra Nevada Ridgeline, & would have to find a passage through. Huntington & Judah traversed the MIDDLE Fork of the Rio de Las Plumas, & emerged beaten, battered, & convinced that no one would ever build a rail line through there. All these years later, it still holds true. And yes, the building of Spring Garden Tunnel, ( and nearby Williams Loop ), allowed Western Pacific to cross over to the NORTH Fork of the Feather River & its tributaries & avoid the lower reaches of the middle fork.
Now, what about the Pit River route, as currently being eyed by Randall Coleman's " Pacific Rim " RR ???