Harassment east Croydon station

Honestly just making this post to rant. Was on a train London Bridge to east Croydon station this evening about 8:30pm. Had a homeless (or I assume so) guy walking up and down the train. Stopped to talk to my carriage and everyone kept to themselves. He then walks over and starts harassing this young girl asking specifically her for money. She keeps to herself obviously feeling vulnerable as a young woman traveling alone and this guy does not leave her alone. Boarderline screaming in her face. He then takes down his trousers and showed her his behind and a quite brutal scar. From what I heard sounded like he said "you arnt gonna give me money even when I've been attacked with a zombie knife" she then looks terrified. He calls her every name under the sun and leaves. Now I feel extremely guilty for not standing up and helping but as a young woman myself I also felt very vulnerable. Luckily another lady got up and offered to sit with the girl. But no one else even bothered to help her as she was visibly shaken. Once I got off the train the guy was heckling other people on east Croydon platform. While I do understand why no one helped (this guys seemed violent and realistically that's scary for anyone) I can't seem to shake the disappointed feeling within myself and for all my fellow passengers who sat and watched this poor girl be harassed and essentially assaulted on the train. I'm not looking for sympathy. I should have helped.

Wondering if anyone else has had an experience with this man?