So what is your question/ comment regards this incident? 
That I can't see why they cant just alerted the train driver by pulling down a security flap thing that they have on trains.
While it is not nice for people going about their day to see a naked man in public, its worse to use violence against what appears to be a man with clearly in a bad mental health state.
The man has now been contained under the mental health act.
They would not act out this way on a female if she did the same thing this guy did.
I am hoping the man gets the help he so clearly needs, this was not done in a sexual manner and so he needs medical help which I hope he gets.
The men that threw some punches and kicks should be held accountable, their excuse cannot be they were protecting other passengers as there don't seem to be any threats or abuse coming from the man.
