Two trains have collided head-on on a level crossing north west of Copenhagen, leaving four people critically hurt and 13 others needing hospital treatment, officials say. The trains were travelling on a line linking the towns of Hillerød and Kagerup in the North Zealand area of north-east Denmark. Tim Ole Simonsen of the Greater Copenhagen fire department could not confirm the cause of the crash, but all the injured were taken to hospital by air or by ambulance. Local mayor Trine Egetved expressed her distress over the accident and emphasized the importance of safety for the many residents and commuters who use the line. Public broadcaster DR shared images of the damaged yellow and grey trains. The Frederiksborg Fire and Rescue service was alerted to the incident shortly after 06:30 local time. While rail accidents are rare in Denmark, experts are investigating whether one of train drivers mistakenly ended up on the wrong line. Egetved stated that such a collision must never happen again.