A teenage boy has fallen off a train on the Sydney Harbour Bridge while reportedly “train surfing” on Thursday.

The horror incident caused travel chaos as multiple emergency services responded.

The Daily Telegraph reports the boy was fighting for life with serious injuries to his head, chest and pelvis.

NSW Police said the teen fell onto the tracks at about 11.30am between the Wynyard and Milsons Point train stations.

“Officers attached to the Police Transport Command (PTC) and Sydney City Police Area Command and have been told a teenage boy had fallen from a train onto the tracks on the Sydney Harbour Bridge.”

A Careflight Helicopter landed in Bradfield Park with a medical team and they treated the boy with NSW Ambulance paramedics.

Police have commenced an investigation into the incident.