Two trains collided early Thursday near Hillerød, about 40 km north of Copenhagen, triggering a large emergency response.
Police described a major accident. The crash happened around 6:30am on a local rail line used by commuters and schoolchildren.
The Greater Copenhagen Fire Department said four people were critically injured and 12 had minor injuries.
All passengers were evacuated, according to North Zealand police. Images showed the front ends of both trains crushed, though they stayed upright on the tracks.
Local mayor Trine Egetved said some injured were flown to hospital. Authorities have not yet given further details on the cause.