The wagons in the above photo are headed east on the old Dutch Flat/Donner Summit Wagon Road. The road was built by the Central Pacific Railroad to bring men and equipment to the construction sites along the rail bed. The railroad was completed in 1869, so these wagons are likely headed up to the summit construction with supplies from Dutch Flat.
Once the railroad was built, traffic slowed on these old roads until the transcontinental Lincoln Highway for automobiles was completed in 1913. The Lincoln Highway often utilized the Dutch Flat Wagon Road through Placer and Nevada Counties, and this is one example. The bridge crossing at Castle Creek is still there, through the bridge is made from concrete, and only carries the curious adventure who utilizes the roads less traveled.
Location at Castle Creek, Soda Springs.