This was a conspiracy theory I always heard growing up in Michigan -- that the auto companies had bought up the trolleys in Detroit so people would have to buy cars. It's good to finally see the historical basis.
"Starting in the 1920s, a company called National City Lines started buying up street car lines, then mostly privately owned. In 1936, the company became a holding company owned equally by General Motors, Firestone Tire, Standard Oil of California, and Phillips Petroleum. Perhaps you can guess where this is going."