Los Angeles had, what was considered, a comprehensive system of trolleys running through the city and the surrounding areas. During a period shortly after World War 2, the street cars were replaced by buses and in 1964 the Southern California Rapid Transit District (SCRTD) was formed.
It was comprehensive, with a myriad of bus types including double deckers, but the drawbacks were that it mostly traveled with the vehicular traffic, except for areas like the El Monte Busway. It didn’t provide it’s own right of way like the current Metrorail system provides. It also was blamed in some circles for adding to the smog problem in the LA basin.
In the 1990s, SCRTD merged with the Los Angeles Metropolitan Transit Authority after the Blue Line LRT from downtown Long Beach to downtown LA and the Red Line Subway line began operation. The successor RTD is now called the Los Angeles County Metropolitan Transportation Authority (LACMTA).