The Los Angeles MTA Metro Rail lines consisted of heavy rail (the fully subterranean Red Line and Purple Line) and light rail (Gold, Blue, and Green Lines) that are a combination of surface, subway, and elevated lines. All use dedicated track right-of-way, not combined use in-road track or electric catenary bus as trolleys and trams do. The LA metropolitan area also is served by the Metrolink commuter system, similar to the San Francisco CAL Train and BART systems. Despite being necessarily car-dependant (as many people live in the exburbs of the Inland Empire) Los Angeles boast one of the best utilized and on time mass transit systems in the nation, and expanding the system is key to reducing congestion. LA has had a number of significant problems expanding the lines, particular the attempt to extend the Red Line out to UCLA, which would require boring under Bel Air and Beverly Hills; legal challenges purporting mythical “gas pockets” have stalled development in that area.
Mass transit in Los Angeles suffers from the same challenges as public transit in any municipality that isn’t so stacked to the gills that only the terminally insane attempt to maintain car ownership, to wit that providing depot-to-destination service is a limiting factor. For instance, I could take the Metrolink to the city I work in, which would only take forty minutes (versus a 30-35 minute commute via car); however, getting from the station to my office via bus would ad over an hour. Nonetheless, when it works, it is nice. Mass transit in LA is clean, efficient, timely, and well-policed. In the occasions I’ve had to take it downtown it is almost always preferable to driving an attempting to park.
As andrewesque points out, the core of Los Angeles actually developed around an extensive streetcar system which was absorbed by and eventually dismantled by National City Lines, alleged (with some justification) to be a front for General Motors. Regardless of the causes, the expansion of Los Angeles metro area stretching from Santa Monica to Los Feliz (with Glendale and Pasadena being the main suburbs) to its present extent going from Ventura to Orange County as far inland as Corona, Riverside, and Moreno Valley doomed a tram system to be inadequate, just as the interstate highway system facilitated such distributed suburban development. Nonetheless, for all the screeching about urban sprawl, Los Angeles is really no worse for its population density than any of a number of large and medium size American cities; Omaha, Cincinnati, and Kansas City, to name a few, have a far worse distribution per density and longer (distance-wise) average commutes.