The system was operating normally again by Monday morning, DSB’s press service confirmed.
The issue impacted online ticketing platforms, machines at stations and staffed ticket desks, although the Rejsekort travel card could still be used on train journeys.
Normal operation was resumed by around midnight.
A DDoS (Distributed Denial of Service) attack is thought to be responsible for the problem. Such attacks bring online platforms down bring placing high demands on them until they are unable to function.
Who or what was responsible for the attack remains unknown, but the press office said it was certain to have come from outside DSB’s own system.
The national rail operator has been the target of hacking attacks before.
In May 2018, a similar issue prevented travellers across the country from purchasing tickets.