Everybody has there opinion clearly however....... From a legal view the guy that everybody is now calling "BIG MAN" has unfortunatly put himself in a position where he can be Sued for assault and battery (Civil) or possibly Prosecuted for assault (Criminal) either way the Teenager would most likely be awarded compensation for his distress and or injurys.
Whilst I certainly dont agree with the Teenagers actions or language at all, at present it would appear that the BTP (British Transport Police) are only concerned about bringing "BIG MAN" up on charges of assault (criminal) Or so the BBC reports, this does not at all suprise me I would personally advise "BIG MAN" would have been of better use if he had simply phoned the police or the ticket conductor had phoned police as they have the power to remove the little Miscreant.
Yet again I will mention I do not agree that the Teenager is in the right I am simply giving my opinion on the legal issue raised.
Ive just read back through all the comments and to clear things up as to the use of "Reasonable Force" it cannot be used as a defence if the person exercising It never had the Lawful right to do so, as in this case the conductor may have used "Reasonable Force" but as somebody has already mentioned he was unlikely to do so because of his age etc... however the "BIG MAN" did not have the legal right to Exercise this power as he has no authority unlike the Conductor.
Somebody also mentions why are bouncers not in court every week well... On private property a bar etc all door staff must be SIA approved and have Legal authority to eject people from the premises using "Reasonable Force" the issue with bouncers is Accountability who stipulates to what degree is "Reasonable Force" an interesting subject for another time.