To follow on on how full a bus (or light rail car) must be to achieve parity with the passenger automobile, consider that for the most part, transit goes inbound in the morning and outbound in the evening, and is hardly used at all between rush hours. In other words, only one fourth of buses or light rail cars have significant ridership, and thus each bus going the ordinary direction during rush hour needs about 40 to 60 riders to merely "break even" in terms of energy usage. A light rail car would need about 100 passengers to "break even" in terms of energy usage and carbon emissions.
An average bus carries 40 passengers, and a typical light rail car might carry 66. The very structure of centralized transit shows that it can never be an environmental benefit when compared with the ordinary automobile.