The girl, travelling in Hyderabad Express, which does not halt at Thane station, tried to alight on platform no 5 of the station from the slow moving train at around 1 PM.
In a bid to get down at the station, the girl fell in the gap between the platform and the train and got badly injured, DCP, Traffic, Rashmi Karandikar said.
Some commuters on the platform raised an alarm and gathered near the train. They tried to click pictures of the victim on their mobile phones, but did not help her even as the train passed by.
Later, with the help of porters he took the girl in an ambulance and rushed her to Thane Civil Hospital, Karandikar said.
The DCP announced that the cop would be rewarded for his gesture.
Earlier also, the constable had chased a chain-snatcher and nabbed him, and had also saved lives of three families during a fire in a building here, she added.