An 11-mile extension of the Metro Gold Line, which starts in East Los Angeles and will eventually go out to Montclair, began in June, and construction is set to begin this year extending the Exposition Light Rail Line from Culver City to Santa Monica.

Taken together, these developments have emboldened mass transit enthusiasts here and lent credibility to what has become something of a legacy project for Mayor Antonio R. Villaraigosa, who ran for office pledging to build a transit system that would upend long-established commuting habits and ease what has long been a bane of life in Los Angeles.

“This put to rest all this talk of, ‘Will we ever build a subway?’ ” Mr. Villaraigosa said, somewhat triumphantly, in an interview. “This is a big deal. People have been talking about it for years. And they were making fun of me: ‘Where is the subway?!’ ”

Los Angeles once had a large, intricate and thriving public transportation system, with so-called Yellow Car trolleys that ran on downtown streets and a vast network of Red Cars, operated by the Pacific Electric Railroad, that ran throughout the region. This was dismantled amid the city’s fervent embrace of the automobile (encouraged, in no small part, by oil interests in Los Angeles that realized the economic potential of the car).

But with a vote by the Los Angeles County Metropolitan Transit Authority’s board last month to approve the Purple Line expansion, there is a consensus that these projects are going to be built, even among those who describe them as a waste of money in a region that will never embrace mass transit. The projects are being financed by a half-cent sales tax surcharge approved by Los Angeles voters two years ago and expected to raise $40 billion over the next 30 years.