Officially, it is not free in order to gauge demand. Personally, I think there is an ideological and pedagogical element. They don't want people to get used to

Officially, it is not free in order to gauge demand. Personally, I think there is an ideological and pedagogical element. They don't want people to get used to the idea that something like free transportation is possible. I heard an interesting talk a couple of years ago about university tuition fees. Germany has fairly low fees compared to other countries. At some universities it is more like a token fee, a couple hundred € per year. Many students get living expenses paid from the state anyway (BAFÖG). It is silly to then pay back some of that immediately. But the thesis of the talk was that the tuition fees were introduced for educational reasons, not to cover the actual cost of teaching. Students were supposed think of themselves as customers, and of education as a good. And they could back this with quotes from neoliberal think tanks. As the FDP (free market 'liberals') is in the coalition, I'm sure one precondition was that the ticket couldn't be free.