Remember that even if the journeys are 'free' in the sense of money, they still take time. And for most people sitting on a bumpy bus isn't as comfortable as sitting at home or sitting in a park (also free ways to spend time).
You should always think of the cost of public transport as ticket price + hourly wage * time spent - value of anything you might get done on the bus.
This is true for busses, but train rides can be quite enjoyable on their own to some people, depending on the train and the view. You still have the opportunity cost of everything else you could be doing, but some people might prefer to read the newspaper on the train, enjoying some shopping a couple towns over and reading on the way back.
You should always think of the cost of public transport as ticket price + hourly wage * time spent - value of anything you might get done on the bus.