Planners foresee the 141-mile segment from Bakersfield to Los Angeles running through vast tunnels, delving through the Tehachapi and San Gabriel mountains, plunging 500 feet underground in some places and soaring over canyons on viaducts 200 to 330 feet high, the Los Angeles Times (http://lat.ms/TxjZ4K) reported.

“It is the project of the century,” said Bill Ibbs, a civil engineering professor at the University of California, Berkeley who has worked on high-speed rail projects around the world.

The $68 billion first phase of the project is expected to run more than 500 miles between San Francisco and the Los Angeles and Anaheim areas by 2029. Eventually, supporters hope for high-speed lines running all the way from Sacramento to San Diego.

Conditions set for the project say it must be able to reach San Francisco from Los Angeles in no more than 2 hours and 40 minutes. The top speed for the Bakersfield-to-LA segment could be 220 mph.

In September, the Federal Railroad Administration approved construction of the first segment, a 65-mile stretch from Merced to Fresno in the Central Valley. Construction is expected to begin next year.

California hasn’t considered such an immense north-south rail link since the 1870s, when Southern Pacific Railroad bored through the Tehachapis. Thousands of Chinese laborers dug and dynamited the way up and through the mountains, creating 18 tunnels on a route that climbed more than 4,000 feet.

Today, only freight trains use the route. Passenger service through the Tehachapis was discontinued in 1971.

The high-speed train won’t be able to use the twisting loops of that route; it will need a straighter, flatter path to maintain its speed.

A corridor for high-voltage lines must be built through the Tehachapis to supply the train, which will need an estimated 2.7 million kilowatt hours of electricity daily, equal to about a quarter of the average daily output of Hoover Dam.

On the way between the Central Valley and the south, the bullet train could zoom across the Mojave Desert, pass through an 8-mile-long tunnel under the canyons of Santa Clarita north of downtown Los Angeles and race through northeastern San Fernando Valley neighborhoods, which may require removing some homes and businesses from the route.

It would go underground near Glendale, run under the Los Angeles River and probably would hit street level again around Chinatown.

The exact route won’t be chosen until next year but about 200 people already are working on the southern segment. Among other things, the tracks will have to cross a half-dozen earthquake faults, including the infamous San Andreas, which could produce a 7.5-magnitude temblor.

The chances of a quake occurring while a train is going over a fault is “very small” but big temblors could cause a derailment, said C. Michael Gillam, a vice president at engineering company Parsons Brinckerhoff, which is overseeing the design of the southern segment.

Ideas being considered are reinforcing viaducts and creating a system that automatically slows or stops trains during quakes.

While the engineering challenges are daunting they are not unique.

Switzerland is building a 35.4-mile rail tunnel under the Alps. And China has a highway bridge 1,627 feet high.