I have read that for Denmark a reservation is required for all trains. However, I cannot make a reservation for the Lindholm-Hirtshals connection, for example. What do I have to do here?
Reservation Denmark (Lindholm- Hirtshals)
+16Which Lindholm? there are several in North Germany and Denmark :)
Trains of DSB (Dansish State Rail) can be done here :)
Note that in Denmark even some private companies like Arriva operate trains. As Arriva operate only regionaltrains (with quite long routes :D ) you don´t need a reservation on their services in Denmark
Oh yes that's right. I am talking about the Lindholm which is close to Aalborg in the very north of denmark.
Okay, so if there is no seat reservation for this route, I don’t have to do anything? I am just wondering because this website https://www.dsb.dk/en/train-ticket-to-europe/ said, that due to covid you need a reservation for all trips.
+16Oh yes that's right. I am talking about the Lindholm which is close to Aalborg in the very north of denmark.
Okay, so if there is no seat reservation for this route, I don’t have to do anything? I am just wondering because this website https://www.dsb.dk/en/train-ticket-to-europe/ said, that due to covid you need a reservation for all trips.
yeah it looks like it. But i can´t reserve the train at the DSB Page online even if NORDJYSKE JERNBANER write it on their page 
Nordjyske JernBaner is the company that operate these trains best option for you is to visit a DSB Ticket office in Denmark forexample Arlborg or Arhus :)
https://nj.dk/
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