If I saw someone push an elderly person whatever the circumstances, then I would intercede.
So if youre on the back of the bus / tram, and the described event is occuring at the front of a busy bus / tram, you would see it with your super X ray vision, and rush to the rescue with your superhuman speed before the punch could be landed?
Go superman!
People trying to push past others happens all the time on buses / crowded places, I strongly doubt that you would have ever seen the punch coming.
Oh wait, yes you would, for you are superman!

Is the personal attack really necessary? Like I said, If I saw something like this happen, then I would intercede, either before it escalated to violence if possible, or by calling the police and attempting to detain the offender if not...by the footage, no-one did anything, with the offender attacking the elderly man and then leaving when the tram arrived at the next stop sithout a single person on that Tram doing anything.........it took children to do something....children!!!
