I just read Jay’s Nov. 25 post on this topic and you should too. The reason most people don’t take buses or trains to work is that they are rational. The time cost of public transportation is exorbitant unless your time isn’t worth much. (Never mind the financial cost, which tends to be exorbitant too, if you take into account, as you should, all of the costs and not just nominal ticket prices.)
And we aren’t even considering the valuable flexibility gained from using automobiles. It’s difficult to run an errand or visit someone on the way home from work when you’re taking the train.
UPDATE: I should have made clear that my comment about financial cost was directed at the newer mass-transit systems in places like DC, Miami and LA. The old systems in places like Chicago and New York are in a different financial category, their fixed costs having mostly been amortized. (The old systems are also probably much more useful to commuters.)