Tehachapi Loop: A Spiral That Solved a Mountain

The Tehachapi Mountains presented Southern Pacific Railroad with an engineering problem in the eighteen seventies: how do you gain nearly three thousand feet of elevation in the distance available before the grades become too steep for a locomotive to climb?

The answer, worked out by engineer William Hood in eighteen seventy-six, was to stop going up in a straight line and go around instead. In a circle, specifically. The Tehachapi Loop is a section of track that spirals entirely over itself, completing a full loop and gaining about seventy seven feet of elevation in the process. A train long enough to occupy the full loop, roughly four thousand feet of cars, will pass over its own rear end as it circles. The lead locomotive crosses above the train's own rear cars a hundred feet below.

The Loop is still in operation. It is, by freight volume, one of the busiest rail locations in the United States. Union Pacific and BNSF run dozens of trains through it daily, every day of the year. The upgrade that Hood engineered in eighteen seventy-six for steam-powered freight trains handles the most modern diesel locomotives without structural modification.

Trainspotters come from around the world to stand at the viewpoint and watch the moment when a long enough train occupies the full circle. The geometry is visible without explanation: the track bends away to the right, curves fully around, and comes back underneath itself. You see the locomotive of the train you are watching crossing above the train's own rear section. The mountain required a corkscrew. Hood gave it one.

The Tehachapi Loop was named a National Historic Civil Engineering Landmark in nineteen ninety-eight, an active piece of infrastructure recognized as a work of engineering history while it is still doing the work it was designed to do.