The driver of the SUV, Josiane Marion, 32, and her two-year-old son, Jacob Lépine, died after a passenger train slammed into the woman's vehicle at a level crossing on the outskirts of Notre-Dame-de-Prairies, 70 kilometres north of Montreal.
There was no barrier at the level crossing on a farm road surrounded by open fields.
The crash happened just before 10 a.m. ET, and police speculated that a thick fog blanketing the area at that time might have prevented Marion from seeing the approaching train.
The force of the train's impact tore the SUV in half.
Investigators from both the Quebec provincial police force and Quebec's transportation ministry spent the day examining the wreckage and the crash scene.
Police spokesman Benoit Richard said they were looking at a number of hypotheses.
"Did she see the train coming?" asked Quebec provincial police spokesman Benoit Richard. "Were the lights open on the railway crossing? Were there bells ringing? And of course, the speed ... of the train [and] the SUV."
Police said Marion was on her way to Joliette from her home in the village of Sainte-Élisabeth, just a few kilometres from where the collision occurred.
The train, with 21 crew and passengers aboard, had just pulled out of the station in Joliette a few minutes before the crash.
Relatives rushed to the scene as soon as they learned of the crash, and some were taken to hospital to be treated for shock.
Police have spoken to one witness and are searching for a second — a woman who apparently stopped and tried to help the victims.