<human>: So I'm on my way back from London on the underground at around 11pm on a busy train, and girl in her early 20's gets on and apparently bumps into this huge black guy sitting near the door. As soon as she sits down he starts shouting at her asking her what she thinks she's doing bumping into him, yada yada yada, she's speechless and just says sorry, he yells some more and she gets up to go to another part of the carriage. As she walks away he swings this mobile phone charger that is randomly wrapped around his hand at her. 

She leaves and he just sits there stewing for a while. All other other passengers are just like 'wtf' and go back to what they're doing. Another stop later and he suddenly gets up and goes straight over to her and starts yelling at her again. 

Suddenly I notice that I'm standing up out of my seat, and then I'm in front of him keeping her behind me. She's crying now and looks terrified, but his attention is squarely on me now. Oh shit, oh shit. I start telling him he's being unreasonable and she's already apologised, etc etc, anything to get some sense through his skull. 

It does nothing. He's obviously looking for any excuse for a fight and starts accusing me of looking at him funny. Everyone else is too stunned to know what to do. Luckily its my stop and I and the girl get off. She's totally in tears and walks with me a bit but gets on a separate carriage.

Big huge black guy follows me yelling all kinds of stupid shit, but I don't turn around and after a while he stops following and leaves me alone. 

The adrenaline is still going, and I'm still amazed because I always told myself if I would never be the one of the bystanders who just keep walking but tonight I found out I wasn't kidding!

Can you condense the text into a few key points?
<bot>: huge black guy tries to start fight with innocent girl and I'm the only one on the train who gets up and stands in his way