LOS ANGELES — When the extension of the Expo rail line opened here in May, it was almost as if the city had stepped into another century. Suddenly, it was possible to go from downtown to the Santa Monica beach by train, escaping a drive that could take two hours. The inaugural runs were packed with people, carrying beach chairs and recording the 15-mile, 45-minute long ride with cellphones.

The $1.5 billion, aboveground Expo project is not the only piece of the transit transformation unfolding here. An 11.5-mile extension of the Gold Line, running from Pasadena to Asuza, just northeast of downtown, opened this spring. If not as glamorous as a train to the Pacific Ocean, it was certainly appreciated by people in the San Gabriel Valley, who otherwise have to navigate the traffic of Interstate 210.

And all of that is likely to be dwarfed by an initiative that is expected to go before the voters in November, pending a final vote by the county Board of Supervisors, that would impose a countywide 1-cent transit sales tax, raising $860 million a year. The tax would finance 40 major transit projects over the next 40 years, including 100 miles of new rail lines and what has been a touchstone for mass transit advocates (and frustrated commuters) for decades: A train tunnel under the Sepulveda Pass, connecting the Los Angeles basin with the San Fernando Valley.

There may be no part of America more identified, for better and for worse, with the automobile than this city. But this burst of activity, and the considerable interest it has stirred here, suggests that a fundamental reconsideration of Los Angeles may be at hand, a shift to an era when mass transit — subways, light rail, buses — could be as central to getting around, and perhaps even to this region’s image, as the car.