I just found an ancient bookmark from 2012 (do you often bookmark interesting things and then never read them?):
https://slate.com/business/2012/09/l-a- ... -city.html"L.A.’s Transit Revolution
How a ballot initiative, a visionary mayor, and a quest for growth are turning Los Angeles into America’s next great mass-transit city.
...
Los Angeles has made this remarkable and underappreciated shift because it has never stopped growing. The core Los Angeles municipality never experienced the kind of postwar population crash that afflicted Northern cities.
In 1940, there were 1.5 million people in the city. Twenty years later, it was almost 2.5 million. By 1990 it was close to 3.5 million. Today it’s 3.8 million and still climbing. The larger metropolitan area has ballooned to 13 million residents, leaving Chicago in the dust as America’s second city. And even though the area is built in a sprawling sunbelt format, the geography of surrounding mountains, ocean, and national forests physically constrains L.A.’s growth. Because of that, the average population density throughout the urban area is actually the highest in America even though the core is much less dense than an Eastern city like New York or Boston. The result was legendary traffic jams, combined with a practical inability to widen the arterial freeways that form the backbone of the city’s transportation infrastructure.
The usual response to too much traffic in the United States is to strangle growth. New development would mean more cars would mean more traffic, so cities adopt rules to block new development.
That’s how San Mateo County between San Francisco and Silicon Valley managed to muster a measly 1.6 percent population growth in the past decade despite enviable access to two of the highest-wage labor markets in America. Over the past 20 years, however, L.A. has chosen the bolder path of investing in the kind of infrastructure that can support continued population growth, and shifting land use to encourage more housing and more people.
..."
The article is seven years old, and a lot of ambitious public transport projects were still in the making then. Would be interesting to find a 2019 update.