I hate to burst some bubbles out there, but what do posters specifically expect the Train Guard or Driver to do?
Any posters coming up with solutions like "The guard should throw them off the train", are pie in the sky, and have no relationship to the real world. We are unable to throw passengers off at unmanned stations (it's against their human rights (sarcasm mode on) - and the rulebook), and if the passenger then decides to "lean" against the train, what then?
The train guard (and/or driver) is on their own, with no personal protection equipment, no training in physical control techniques, no training in physical combat, no legal ability to do so in anything other than self defence (and even then it is open to the CPS to decide was it justified or not, and all that involves), and they have very specific written instructions that under NO circumstances are they to place themselves in any situation where they are personally in danger.
Every time we do intervene with passengers like those described by the OP, we are in breach of the "Violence At Work" policy for approaching a potentially dangerous situation and not locking ourselves in the back cab.
TOC's are fully aware that train guards, and drivers on DOO trains are highly vulnerable lone workers, but instead of adopting an approach that would ensure more resources are given to ensure our safety, they have responded by writing policies and procedures where if a guard is assaulted, the TOC's legal liability is minimised by them being able to say that the member of staff was not following company policy.
You may have noticed I have not mentioned BTP yet. That is because they are an irrelevance. Essentially, all BTP are any use for is taking statements after the event (preferably 9-5), and looking at the CCTV pictures to see if they can manage to identify anybody. In terms of dealing with the actual trouble, they are not in the equation. It is now 22.30 on a Saturday night, and I would be surprised if there are enough BTP officers on duty in all of South Wales for me to need more than one hand to count them on. Past observation would confirm that suspicion.
So, would a guard make a difference? Probably. Would a guard intervene? Probably. When they get thumped will management say it was their own fault? Probably.