I've posted about this before but earlier this year I thought I felt someone gently tug on my hair from behind.
I was in a lift with a few other people at the time. When I turned around none of them seemed to be looking at me or paying me any attention at all.
The next day I realised that someone had actually cut off part of my hair and after posting about it on here I was convinced enough to mention it to the community police. They viewed the CCTV and could see a man reaching up behind me and me turning to look at him, although they couldn't get a clear look at him or what he had in his hand to cut my hair with.
I wish I'd shouted out the way this girl did and drawn attention to the man behind me. I am assuming she shouted immediately in shock or pain before she turned and realised it was a toddler pulling her hair. It would help if the OP/her DH could clarify though as it seems to be the sticking point of the thread.
I didn't shout because it was just a gentle tug at my hair, and once I'd looked around I thought I'd probably just caught it on my coat hood button or something. I wish I'd listened to my first instincts of thinking my hair had been pulled.
It upset me a lot when I realised what had happened, especially when I realised that people who cut women's hair usually escalate their behaviour to worse attacks on women.
I know that's not what was happening on your train, but in the first instance of the girl feeling someone yank her hair, and your description of it makes it sound like it would have been fast and painful, I can understand why she was alarmed and possibly frightened for those few seconds where she shouted and before she turned around to see who had grabbed her. She probably didn't know who had grabbed her or why when she shouted out.
It's obviously not your baby's fault. He's only 15 months old and as long as you are telling him 'no, that hurts' and stopping him when he pulls someone's hair he will soon learn not to do it.
Your DH was the unreasonable one. He knows your son has this habit, he was in charge at the time and he was aware the seats were back to back so your standing up DS was within easy reach of the girl behind him.
The young was probably startled and hurt, cried out in shock, realised she had shouted at a baby, received no apology from your DH and feeling embarrassed that she had shouted out on a busy train and had had everyone look at her.
Your DH could and should have apologised to her at the time regardless of what he thought she might say in reply.