A full bus or trainload of people is more efficient than private cars, sometimes quite a bit more so. But transit systems never consist of nothing but full vehicles. They run most of their day with light loads.Yeah, the local transit always seems mostly empty whenever I see it. But
...it is always the green move for any individual to take existing mass transit over their car. That's because the transit is running anyway, so the incremental cost of carrying one more passenger is indeed less than just about any private vehicle. It is similarly green to carpool in somebody else's car that's going your way.