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The Pengra Pass rail route, also known as the Natron Cutoff, the Cascade Subdivision, or the Cascade Line, is a Union Pacific Railroad line (originally a Southern Pacific Railroad line) connecting Eugene, Oregon, with Klamath Falls, Oregon. Construction of the line began in 1905 and was completed in the mid-1920s. Its name denotes a mountain pass on the Lane County–Klamath County boundary in the Cascade Mountains of Oregon, about 1.5 miles (2.4 km) from Willamette Pass. The line heads southeast from Eugene, up the Cascades and over Pengra Pass, then southward beside U.S. Route 97 to Klamath Falls, where it splits in two, each track continuing into California. The route has at least 22 tunnels, several snow sheds and multiple bridges across canyons.
The Pengra Pass route was built to provide trains running between Eugene and California with a cheaper and otherwise more practical option than to follow the older Siskiyou Pass route, which runs south from Eugene and over Siskiyou Pass to California. As a result, the Siskiyou Pass route was rendered nearly obsolete. As of 2011, no trains had used the Siskiyou Pass route in almost a decade.
Nonetheless, a major helper operation is demanded by the Pengra Pass route's 44 miles (71 km) of constant grade—the longest anywhere on the former Southern Pacific system, including Donner Pass.
The right-of-way is surrounded by waterfalls and streams running through a Douglas fir forest. At many locations, the track runs atop the steep southern slopes of the Salt Creek canyon. For hours before one sees a train on the route, one may hear the noise of multiple-unit diesel engines thundering up the canyon walls in their task to get the trains over the mountains; in the other direction, dynamic brakes whine.