> At the extremes, a bus (or worse, a train) with one driver and one passenger is obviously worse than one person in a car.

Or driving with no passengers at all, not uncommon for the buses where I live at certain times and routes.

It's sort of a chicken and egg problem. You can optimize equipment usage by running more buses on heavily used routes but you can't encourage new ridership on new routes without running enough buses to make it at least somewhat convenient. If you miss your bus and have to wait an hour or more for the next one, most people don't find that very appealing.

But at some point if you're running a whole bus to move a small number of people, you need to admit it's not worth it and eliminate that route. It would be more economical to just give those people taxi rides.