The prosecution remains reserved under Art. 20 Passenger Transport Act (PBG), that in addition to the payment of the fee as compensation for the administrative effort and the loss of revenue, the traveler must additionally expect prosecution. The relevant penal provisions can be found in Art. 57 PBG and in the Criminal Code.
The punishment comes from the criminal field of your journey without a valid ticket. The amount is determined by the prosecutor and owed to the law enforcement authorities. A non-payment may be subject to imprisonment.