The unidentified bodies of the duo were found dumped on railway tracks near Pali village between Mumbai and Nashik on Monday evening, they said.
"It was revealed in the postmortem report of the deceased that the infant was strangulated to death while the woman's head was smashed with stone," assistant inspector Bapusaheb Shinde of Shahapur police station said today.
A case under section 302 (murder) of Indian Penal Code (IPC) was registered against unknown persons, Shinde added.