Clearly I've no idea what happened here but it reads similarly to an incident I witnessed on a train out of Waterloo, and ending at a major outer suburban station, around 20 years ago.
Firstly, you don't sit watching the behaviour and interaction of every passenger in a crowded carriage. I was reading and was vaguely aware of what sounded like an odd conversation several seats away from me, but you overhear a lot of odd things on trains.
As the train approached the next calling point, I was aware that the offender quickly left the seat and was ready to alight from the train as soon as the doors were released. As he did so, but not before, the victim, who was also travelling to the same station, became visibly and audibly upset and it was only then that I understood what had happened. I was also getting off so, with others, I asked her if I could help. I accompanied her to find the guard, who held the train until the police arrived - fortunately this was a station with a BTP office - and we went to view CCTV coverage. I don't recall a conversation about CCTV on the train itself, there probably wasn't in that era. We quickly found a clear image of the offender leaving the station. I provided a witness statement and left.
I wasn't given the opportunity to intervene earlier but if I had, I would've had to consider the possibility of the incident becoming violent rather than physical.
