The impact ripped the truck in half, leaving debris scattered around a nearby field.
The 32-year-old mother and the two-year-old child were transported to hospital where they were declared dead.
Quebec provincial police spokesman Benoit Richard said the collision happened near Notre-Dame-des-Prairies, not far from the Joliette train station.
There were warning lights but no barrier at the crossing about 50 kilometres from Montreal.
Richard said fog may have been a factor.
The train carrying 21 passengers had left the station about six minutes before impact.
No passengers were injured and the woman and child were the only people in the SUV.
The federal Transportation Safety Board also sent a team to the scene of the collision to investigate.