Wow, what a weird thread. Having just completed a four hour journey on a massively overcrowded train, I'm just in the mood to comment.
First off, I don't think there's anybody in the world who likes the idea of other people's children on public transport. They are more or less guaranteed to be the most irritating little brats you've ever had the displeasure of coming across... and I'm a teacher, I've a high threshold for annoying children! However, far worse are the wet, and yes, useless parents who do sweet FA to control their children. I had my foot trodden on, on purpose, and an attempted trip-up by an oh-so-adorable toddler today, whilst oh-so-busy mummy was fiddling away on her blinking iPad.
So, OP, your toddler shouldn't have been stood up on the train seats at all. I don't care if it's a 1.5 hour journey, they should be sat down with a book, colouring, small toy, whatever. NEVER stood up on the bloody seat. It's annoying to everyone around you, who immediately has to keep an eye on the child to make sure they don't get grabbed or vomited on, etc.
Therefore, you may well BU for having earlier allowed your toddler to behave in such a way before you went to the loo. Especially knowing he's a hair-puller by toddler trade.
Your husband is obviously being massively unreasonable for allowing the event to happen (you are exempt from the actual incident, as you weren't there), but most especially for not apologising for it. What a tit! I am sure you've told him so.
However, his "fear" of a teenager's rage is nothing compared to the fear of a young woman who suddenly thought herself being assaulted in public, and rightly yelled out in an attempt to stop it. There's never going to be anything unreasonable about a person who attempts to stop a possible assault by calling out in alarm. Many of us have been assaulted in public, it's happened to me numerous times, most recently just last month. Young women should be taught how to protect themselves from this in public, as it seems that, unfortunately, many young men are still not learning the lesson to keep their hands and other various parts to themselves and away from strangers.
The girl was right to shout, never mind what she shouted, she won't have damaged your son in anyway. Your husband was awful to not apologise for the incident. She may well have really thought it was him, not the baby.