British Transport Police were called to the Beech Hill crossing on Springs Road near Finningley at 12.35pm today.
A spokesman said emergency services were at the scene, where the train is upright and on the tracks.
Fire crews from Harworth, Retford and a specialist rescue team from Tuxford attended the scene.
When they arrived, the woman and child were already out of the car.
A child, believed to be around three years old, was seriously injured and air lifted to hospital.
A 76-year-old woman was taken to hospital by ambulance with minor injuries.
No-one on the train was injured. Firefighters helped evacuate around 20 passengers from the train.
Rail replacement buses are running between Doncaster and Lincoln Central, adding 45 minutes to journey times.
Check here later for more details.