Southwestern Ontario police continue to probe the death of two young children after a van carrying a father and four kids was struck by an oncoming train.

Wynter Williams, 6, and her three-year-old sister, Brooklyn, were killed when a CP freight train smashed into the family's van outside the town of Lakeshore on Sunday morning.

Their brother Dryden Williams, 4, remains in a Detroit, Mich., hospital clinging to life, while the children's father, Andrew Williams, remains in a Windsor hospital with serious, life-threatening injuries.

A fourth child, one-year-old Jasmyn Williams, received minor injuries.

The town of Lakeshore is on the southern edge of Lake St. Clair about 30 kilometres east of the city of Windsor, near the U.S. border.

Police said the catastrophic collision occurred on Strong Road, north of Country Road 42, at about 10 a.m. when the southbound van was struck by the train at a level crossing.

CP Rail police are working with provincial police to determine the cause of the crash.

CP Rail said visibility at the crossing was good, and the train's crew tried to alert the van's driver.

"The crew did everything within their means to alert the driver of the oncoming train by sounding off their whistle and then eventually putting the train in an emergency brake application," said CP Rail spokesperson Kevin Hrysak said in a statement.