From PG: A blogpost by Brad Templeton: Is green U.S. mass transit a big myth?
... These studies express transit energy efficiency in terms of BTUs per passenger-mile. The BTU is the English system unit of energy, and it's equal to 1055 joules. On pure conversion, there are 3413 BTUs in a kw/h. To turn BTUs/mile into miles per gallon, you divide into 125,000, the number of BTUs you get from burning a gallon of gas. ... The DoE figures describe the average car as using 5500 BTUs/mile (23mpg) or 3,500 BTUs/passenger mile with an average load of 1.57 passengers. This is a "fuel to wheels" number based on burning the gasoline.
Putting the car at 3,500 I was disturbed to learn that city diesel buses and electric trolley buses are both mildly worse than the car in energy efficiency. Light rail systems are also slightly worse, on average, though it varies a lot from city to city. Commuter rail and subway (heavy rail) trains tend to be a bit better, but not a lot better. (Non-hybrid cars are also better at long haul than they are short haul.)
Templeton is basically right, I have seen this data before, and we make basically the same argument in The Transportation Experience (Chapter 19). (A car with 4 passengers would be much much better, since the metal per person in a car is much less than on transit, of course cars generally have less than 4 persons most of the time).