The 73-year-old man was allegedly punched in the face by the teenager at Caringbah station, with the force of the blow knocking him to the ground and causing bruising and swelling to his jaw.

The man had been travelling on city-bound train from Cronulla on Saturday afternoon, when he spotted a boy exit the train at Caringbah station and allegedly graffitied a platform wall just after 5pm on Saturday.

The 15-year-old boy was part of large group of teens who were travelling on the same city-bound train.

As the man attempted to stop the boy, another 16-year-old boy left the train and allegedly punched the older man in the face.

The punch felled the man, and the teenagers scurried back onto the train, rejoining their group of friends.

But Police Transport Command officers were quickly alerted to the incident, and the16-year-old was arrested on the platform.

He was taken to Miranda Police Station and charged with assault occasioning actual bodily harm, common assault and an outstanding warrant and was refused bail to appear at a Children's Court on Sunday.

Police said the 15-year-old boy would be issued with a court attendance notice for a graffiti offence.

Another 16-year-old boy, was allegedly found with two knives in his possession. The knives were confiscated and infringement notices issued for the possession of knives in public place.

The older man was bruised and swollen around his jaw, and grazed his elbow in the fall, but did not require medical treatment, police said.