Four children were killed and two people seriously injured in collision between train and cargo bike, popularly used by Dutch parents to transport their children, the Dutch police said.
The accident took place in the eastern city of Oss, near the German border around 110 kilometres (65 miles) southeast of the capital Amsterdam.
A police statement said a "very serious accident" had taken place with "gravely wounded and possibly multiple fatalities."
"It is really a terrible accident," said a spokesman for the national railways service NS.
Police confirmed that the train from the eastern town of Nijmegen to nearby Den Bosch collided with the three-wheeler bike which was carrying children.
"Four children were killed in the accident. Emergency services are still at the scene," police in Brabant province said on Twitter.
A woman and another child were seriously injured.
Cargo bikes — bikes with a large box on the front used for carrying children or shopping — are a common sight in the Netherlands, where cycling is a way of life.