A mother and her three-year-old son died after they were hit by a train in a "tragic incident" during the morning rush hour, police said.
Donna Oettinger, 41, and Zachary were hit by the mainline service after Ms Oettinger was reportedly seen dragging the child on to the tracks.
No-one else is believed to have been involved in their deaths at Riddlesdown rail station near Purley, south London.
Detective chief inspector Iain Miller of British Transport Police (BTP) said: "Our thoughts remain with Donna and Zachary's family and friends at this sad and difficult time.
"Officers are continuing to work to establish the full circumstances of what happened, although early indications show that no one else was believed to have been involved. A file will be prepared for the coroner."
Ms Oettinger and her son were hit by the 7.30am service from Uckfield in East Sussex to London Bridge. The train had been travelling at around 60mph and was not due to have stopped at Riddlesdown station.
BTP said it was called to the "tragic incident" at 8.29am, along with the Metropolitan Police and ambulance teams. But medics were unable to save the pair who were declared dead at the scene.
Officers could not confirm reports that Ms Oettinger was seen pulling Zachary into the path of the train.
The station was temporarily closed following their deaths and trains were prevented from running on the line for several hours after the electrical power between Woldingham and South Croydon was turned off.
There was considerable disruption to services run by the Southern train company in the area, with buses running between some stations and extended journey times. The line between Uckfield/East Grinstead and South Croydon later reopened.