The boy and mum were standing on a platform at Frankfurt’s Central Station when they were shoved in front of a train, say police.
Horrified passengers, including several children, looked on, with one witness saying: “People collapsed howling.”
The high-speed train from Dusseldorf to Munich hit the boy as it pulled into the station at about 10am local time today.
The boy was run over, but his mum managed to escape and was taken to hospital.
German cops are treating the incident as a suspected homicide.
A 40-year-old man, originally from Eritrea in Africa, has been arrested.
Police said there appears to be no connection between the victims and the suspect, who allegedly fled the scene and was arrested after being chased by commuters.
There are reports he tried to push a third person in front of the train.
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