A dismissed Central Reserve Police Force constable, Hemraj, threw a 14-year-old boy out of a running train on the Alwar-Rajgarh section of the North-Western Railway, police said in Jaipur on Wednesday.
The incident occurred on Tuesday on the Hissar-Jaipur shuttle.
The boy was taken to a hospital and his condition was said to be serious, the police said.
The Railway Police arrested Hemraj and investigations were on.