Transport authorities are warning of a recent spike in a dangerous social media craze after a teenager fell onto railway tracks while reportedly train surfing in Sydney's south.

Emergency services were called to Caringbah Station after reports a 13-year-old boy had fallen off the back of a train onto the tracks about 6pm on Monday night.

The teenager was treated at the scene for head and arms injuries before he was taken to Sydney Children's Hospital in Randwick, where he remains in a serious condition.

Two other boys, aged 12 and 13, are assisting police with their inquiries.

Police suspect the injured boy was train surfing, which is the act of riding on the outside of a moving train.

Coordinator general of Transport for NSW, Howard Collins, happened to be on a nearby service when th

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