Newhall Auto Tunnel, South Portal, ca. 1920s. Real photo postcard by Mission Art Company of Los Angeles.

As the age of the automobile descended on the Newhall Pass, Beale's Cut proved to be too difficult a climb for this new mode of transportation. Therefore in 1910, the 435-foot-long Newhall Auto Tunnel was constructed a quarter-mile to the northwest of Beale's Cut. Just 17½ feet in width, the tunnel was quite narrow, making it difficult for two-way traffic to pass through.

By 1938, with increasing auto and truck traffic, the California Division of Highways determined it needed to be replaced. In July of that year, the mountain above the tunnel was blasted away, and a four-lane road was built above the level of the old tunnel. It was first known as Highway 6, then Highway 14, and finally as the present-day Sierra Highway. The cutout of the mountain where the tunnel was located can still be seen today on Sierra Highway at the crest of the Newhall Pass.