Prepare to be bored...
From the Railway Byelaws:
?(2) Removal of persons
(i) Any person who is reasonably believed by an authorised person to be in breach of any of these Byelaws shall leave the railway immediately if asked to do so by an authorised person.
(ii) Any person who is reasonably believed by an authorised person to be in breach of any of these Byelaws and who fails to desist or leave when asked to do so by an authorised person may be removed from the railway by an authorised person using reasonable force. This right of removal is in addition to the imposition of any penalty for the breach of these Byelaws.
The boy who threw the guy off asked the authorised person, who gave permission. I think that made him an "authorised person".