If I did stand up to the group of idiots promoting murder, and a fight did erupt, could I be liable to be charged? Problem with this country is that the law often seems to pick on the victims and do-gooders more than the thuggish.
As a result of this, I would honestly be disgusted, but act as normal for a train journey and ignore loud shouting people, and just talk about how annoying they are to sensible passengers nearby. You can't really stop idiots shouting, asking them to stop would result in more shouting, these on your train were likely trying to upset the public on purpose, as to get a confrontation.
I always try to ignore such news stories, I believe that the press generates a type of legendary status for murderers, but not for the victims or crime solvers, that inherently causes idolisation of killers and copycats, especially when nicknames are thrown in.