Uh, the other (correct) way to look at it is the Bay Bridge requires a staff of 1000 people to move 1000 people across the Bay. Just because you externalize the costs doesn't mean you eliminated them.
Some people might get work done on a train, but not many. Most just waste their time.
So those 1000 people spent that time either way, makes little different if they are passengers or drivers. However driving gets you there faster, so the costs are lower.
So those 1000 people spent that time either way, makes little different if they are passengers or drivers. However driving gets you there faster, so the costs are lower.