It is not free, you need to have a ticket, if not and you get controlled, you pay a hefty fine (>100€) which discourages fare evasion and compensates for the ones that still do it. There are no turnstiles, which of course saves a huge amount of money and also avoids queues in the Stations.

Kennt man vom Bogen schießen, den hält man abhängig vom dominanten Auge, nicht von der Hand