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Dartmouth Engineer - The Magazine of Thayer School of EngineeringDartmouth Engineer - The Magazine of Thayer School of Engineering

Inventions: Tehachapi Loop

By Lee Michaelides

Inventor: William Hood, Dartmouth Class of 1867

William Hood
William Hood. Photograph courtesy of Dartmouth College Archives.

Dartmouth produced engineers of national importance even before the founding of the Thayer School. William Hood, for example, was the engineering genius behind California’s Tehachapi Loop, one of the seven wonders of the railroad world and a National Historic Civil Engineering Landmark.

When Hood joined the Central Pacific Railroad after graduation in 1867—just as Thayer School was being established—the company was a startup with only 90 miles of track. By the time he retired 51 years later as chief engineer of the Southern Pacific Company, which had absorbed Central Pacific, more than 11,000 miles of track had been laid, much of it running through rugged Western mountain ranges.

Robert Fletcher, dean of Thayer from 1871 to 1918, described the Tehachapi Pass between San Francisco and Los Angeles as “a bewildering labyrinth of lofty peaks and ridges where the roadbed twists and squirms by every sort of horseshoe curve, S curve, and spiral.” Hood’s loop is the crowning glory of the 28 miles of rail line that snakes through the mountain pass. The elegant .73-mile spiral alone ascends at a 2-percent grade for an elevation gain of 77 feet. A train longer than 4,000 feet—some 85 cars—passes over itself as it travels along the extraordinary layout.

Tehachapi Loop in the mountains between San Francisco and Los Angeles
CIRCULAR LOGIC: William Hood’s Tehachapi Loop in the mountains between San Francisco and Los Angeles was completed in 1876 and still carries up to 40 trains a day. Photography courtesy of

Hood’s spiral became an industry standard. It was incorporated into Thayer’s railroad curriculum, and Hood personally prepared teaching materials for the school. He also offered up this piece of advice: “The essence of engineering consists not so much in the mere construction of the spectacular layouts or developments, but in the invention required—the analysis of the problem, the design, the solution by the mind which directs it all.”

Dean Fletcher surmised that Hood’s training in descriptive geometry at Dartmouth was key to his success because descriptive geometry was to the engineer what the study of literature was to the poet: It “compels the man to be exact and true.” The same can be said of the Tehachapi Pass, with its 18 tunnels, 10 bridges, and Hood’s remarkable loop.

Categories: Inventions

Tags: alumni, design, history, innovation

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