If you don`t have a valid ticket you may be charged a penalty fare (§ 12 German Railway Regulations (EVO)).
The fee is 60 Euros in metronom trains, no matter which route you choose.
You can pay the fee in cash by the passenger advisor in the train (just in the moment when`s happening) or via bank transfer (you can find the needed information on the penalty fare itself).
In the latter case your personal data will be recorded.
This form is also usable to present a valid and personal season ticket.
Some more information around the penalty fare
Operating a rail network costs a lot of money. Staff wages, acquisition and servicing costs for the trains, track access and station usage fees, cleaning, safety and informational materials must all be paid for. Selling tickets only allows us to cover around 30% of these costs. The rest is subsidized by the state of Lower Saxony.
Everyone who doesn`t buy a ticket is therefore increasing the share of public tax revenues that are needed.