Two other people were injured in the accident in the city of Oss.
Four children were killed and two people injured in the Netherlands on Thursday when a train struck a "cargo" bicycle used to ferry children around.
The accident took place in the south-eastern city of Oss, some 100 kms from Amsterdam, at 8.20am at a rail crossing equipped with warning lights and red and white barriers to keep pedestrians off the track when a train approaches.
Two of the children who lost their lives were 4 years old, the others were aged 6 and 8.
Police revealed that three of the children involved in the collision are from the same family. One of them, 11, survived, along with a 32-year-old woman. Both are in hospital in critical condition.
Police said the families of those involved have been contacted.
They added that the electric cargo bike belonged to a daycare centre and the injured woman, an employee, was taking the children to school when the accident occurred. Several people witnessed the accident, police said, "including several children".
The Oss mayor, Wobine Buijs-Glaudemans, said in a statement the local community is "extremely upset".
"This morning we were shocked by the accident on the track with a cargo bike with children. This affects us all," she added.
ProRail, the state-owned company in charge of rail infrastructure in the Netherlands, said that an investigation had been launched to determine the cause of the accident.
"This is a very dark day, terrible for the next of kin and those involved. There are no words to describe this," CEO Pier Eringa, said in the statement released on Twitter.