I don't know about the situation in LA, but nationwide this process is documented fact, not conspiracy theory. In Baltimore, the streetcar system was bought by private hands connected to tire moguls (I believe specifically it might've been Goodyear, but I'd have to double-check on that) who then dismantled the whole thing after a few years of pretending to operate it in good faith.Not to get all conspiracy theorist but as someone who grew up in a town basically created around the fortune of the Huntingtons (who ran THE major railway line, not Pac Elec though, in Los Angeles), I heard that they were paid off by various auto-related lobbies to let the railways die so they could pave the way - literally - for more auto sales and freeway development and suburban housing growth.
I think one of the major proponents against rail transit in the early 1900's was standard oil, basically a large conglomerate that was dissolved by the supreme court, later becoming a bunch of subsidaries like exxon mobil and shell. If i remember correctly there were plans for a nationwide rail system that they had nixed in order to build the american highway system.