It’s a cliché to say Southern California has never known this, although like most clichés, it has some basis in truth. As the writer Louis Adamic noted in the 1920s, the single-family home, with its emphasis on private life, is Los Angeles’s essential building block. In the resulting landscape, social life occurs behind closed doors. Taken to its most extreme form, this becomes the Los Angeles described by the urban theorist Mike Davis as “fortress L.A.” — “where the defense of luxury lifestyles is translated into a proliferation of new repressions in space and movement,” and street life takes place only in a car.

Los Angeles, however, is contradictory: a city of sprawl, but also of neighborhoods; of glitz and glamour, but also of the most pedestrian realities. If I wander even just 10 blocks west from my home, I can see several subtle shifts in character, Little Ethiopia blurring slowly into the quiet residential streets of Carthay Circle, and then into the Orthodox Jewish community of Pico-Robertson.

IN recent years, there has been a return in Los Angeles, as in many other cities, to the urban core. (Sprawl, it turns out, cannot occur indefinitely.) The residential population of downtown has nearly tripled since 1998. Walk along Main Street or Broadway here on a Saturday evening and the atmosphere is that of a city reconstituting itself. Density, verticality, sidewalks crowded with pedestrians: This is public space, a sense of the street as a shared landscape in which we have no choice but to accept that we are here together. It’s Los Angeles not as fortress but its opposite: community.

“We’re not building for today,” Phillip A. Washington, the head of Metro, told The Los Angeles Times when asked about the declining numbers of riders on buses and trains. “We’re building for 100 years down the road.” That may sound like the rhetoric of public relations, but I think he has a point. In 2013, a designer named Nick Andert created a map of what the Metro system might look like in 2040. It features a dozen lines and hundreds of stations, most as yet unbuilt, “extrapolated from Metro’s concrete short-term plans and somewhat more vague long-term plans, along with a little guessing and creative license.” Wishful thinking? Possibly. A kind of civic parlor game? Without a doubt. Still, the word that most comes to mind is vision: a strategy to consider the city through its most expansive possibilities.

In that sense, the Expo Line (indeed, all of Metro) may be most valuable for how it encourages us to think about the future. The knock on Los Angeles has long been, to borrow Mr. Adamic’s ruthless formulation, that it “grew up suddenly, planlessly.” If the assessment is not untrue — how could it be otherwise, in a city where the population jumped from 100,000 to 1.2 million between 1900 and 1930? — it also doesn’t matter any longer. Like the Red Car, 1930 was a long time ago. More important is where we go from here.

Certainly, Los Angeles will always be a city of the car. But it may also become, is becoming, a city of the walker, of the rider, in which the streets are not only a terrain we pass through, but also one we actively share. Regardless of what the Expo Line ultimately does or doesn’t do for traffic, this is, I think, the essence of what it offers: the notion of Los Angeles as a space we occupy together, collective and evolving, where in the act of getting lost, as I discovered on Venice Boulevard, we may also unexpectedly be found.