[joeysteele]

I think the big guy was right to help get things back on track after asking the Conductor if he wanted him off the train, once the Conductor said yes to that he invited the bigger guy to help remove the teenager from the train.
The Conductor handled this very badly.

However, not having a valid ticket and then being abusive,loses you your right to even be on the train and technically this teenager was affecting the time and lives of everyone else on the train.

A court if it gets to court, would take all that into account, that had the teenager had a valid ticket and presented it then he had every right to be on the train, rail staff have a right to go about their jobs properly without being abused in the process.

So in law, the teenager had then no right to be on the train in the first place and secondly he would not leave the train when requested to do so by the legitimate Conductor.

Unfortunately,in law to manhandle anyone is assault so while in law it would be seen that an assault was commited by the big guy.
However, a court would likely also take into account that provocation was evident from the teenager in his refusal to leave the train and therefore cause massive inconvenience to a great many other people and that in this instance the end justified the means.