A 14-year-old boy has been charged over throwing a seat out of a train in Sydney which landed on a motorway.

A video posted online shows the teens laughing as the seat lands on a busy motorway below in Sydney's North Shore.

The boy's mother saw the video then took him out of his 14th birthday party and straight to police.

Police relased a statement saying the boy has been charged.

'He was charged with two counts of enter crew compartment of train without permission, protrude part of body from open door/window on train, throw thing from public passenger vehicle/train and throwing rocks and other objects at vehicles and vessels,' they said.

He will appear in Children's Court on April 4.