The crash, involving a Via Rail train, occurred just before 10 a.m. near Joliette, Quebec, the Canadian Broadcasting Corp. reported.
Quebec provincial police had not identified the victims, but said the woman was in her 30s and the child was under 5, possibly a mother and her child.
Sgt. Benoit Richard said they were taken to a hospital in critical condition and later pronounced dead.
Richard said the crash occurred in heavy fog at a crossing with no barrier, but the circumstances of the accident were still being sorted out, the CBC said.