A Danish passenger train hit falling cargo from a passing freight train as it crossed a bridge, killing six people and injuring 16 others.
The accident was the worst in Denmark in more than 30 years.
The rail operator, Danish Railways, told Denmark's TV2 that the victims were passengers on a train going from the city of Odense, on the central Danish island of Funen, to the capital of Copenhagen when the accident took place about 8am yesterday.
Authorities said the trains were going past each other in opposite directions.
Aerial TV footage showed one side of the passenger train had been ripped open.
Photos showed that the freight train was carrying crates of beer, and the tarpaulin that covered the cargo train was torn in pieces.
Police declined to comment directly on a report from TV2 that a large freight container was likely to have fallen off the cargo train.
Jesper Nielsen, who was on the passenger train, told TV2 the train "was out on the bridge when there was a huge bang ... very quickly thereafter, the train braked".
The accident took place on a road-and-rail bridge, part of the Storebaelt system of bridges and a tunnel that link the Danish islands of Zealand and Funen.
The transport system was closed to cars overnight because of strong winds but trains could pass.
Road traffic resumed yesterday with a 50kmh speed limit.
Police spokesman Lars Braemhoej said that while "we do not know precisely what caused the accident", one possible cause was that cargo from the freight train fell off and hit the passenger train.
He added there was "considerable damage" on the passenger train.
"Ordinary Danes on their way to work or heading home from the Christmas holidays have had their lives smashed," Prime Minister Lars Loekke Rasmussen said as he issued his condolences.
Police urged passengers to contact relatives and tell them if they were safe and urged people not to share photos or videos of the accident.
Flemming Jensen, the CEO of state-owned Danish Railways, said police and the Danish Accident Investigation Board were investigating the damages.
He said the operator "will contribute everything that we can to the investigation",
Bo Haaning of the Danish Accident Investigation Board was quoted as saying it could take months before the cause of the accident could be determined.
Kasper Elbjoern, spokesman for the Danish brewery group Carlsberg, confirmed that a freight train transporting its cargo was involved in the accident.