Train on rail loop in the sunrise light. Tehachapi, CA USA

Observers from around the world come to Tehachapi specifically to witness the most heavily trafficked single-track main line in the country and the engineering marvel that is the Tehachapi Loop.

Along its winding path up the northwestern slope of the mountains, the track makes horseshoe curves at Caliente, Allard, and Keene, and passes through 18 tunnels.

At the Loop, the line climbs in a spiral over itself gaining 77 feet of elevation. This engineering feat has been named one of the railroad wonders of the world.

In 1955, a monument celebrated the Loop's designation as a California Historical Landmark, and in 1998, it was honored by the American Society of Civil Engineers as a National Historic Civil Engineering Landmark.

The 68 miles of track between Bakersfield (elevation 400 feet) and Mojave (elevation 2,700 feet) crests the summit at 4,025 feet.

The 2.2 percent gradient is maintained downhill to Mojave. Over the intervening years, 36 miles of the line have been double-tracked and a project will begin soon to double-track more of the line between Tehachapi and Bakersfield.