Williams Loop

The Williams Loop is a rail spiral located on the Union Pacific Railroad's Feather River Route through the Sierra Nevada mountains in northeastern California, connecting the Sacramento Valley to Salt Lake City via the Feather River canyons. Located approximately five miles east of East Quincy, the loop is used to gain elevation on the WP's eastward climb to its summit at Beckwourth Pass while maintaining the railroad's overall 1.0 percent grade, the least steep of any grade on a transcontinental railroad. More specifically, the loop helps the railroad to surmount the divide between the East Branch North Fork Feather River and the Middle Fork Feather River.

The Williams Loop is part of Plumas County's "7 Wonders of the Railroad World" and access is described in its travel guide.

Wikipedia