> Even if you only take 2-3 trips ...
Yeah, and better do not forget to cancel those because the friendly DB automatically prolongs these for another year.
In case you refuse to pay, they send a friendly debt collection service -- Infoscore, which conveniently also acts as a credit rating agency.
If you still refuse to pay, the "independent" law firm Haas & Kollegen, which magically resides in the same building as the "independent" debt collection agency and credit rating agency, will take you to court.
Did I mention that these three entities are totally independent, as required by law?