Reprimanding a stranger in public?

This has been bothering me since last weekend, but I haven't had a chance to put all of my thoughts into words until now.

I was sitting on the 6 train, going to Grand Central quite early last Saturday morning, desperately trying to memorise my lines for the part I was playing in Miriam's short movie that day. A young girl and her brother were bickering beside me, when all of a sudden their mother (who was sitting across from us) yelled at the them to "shut the FUCK up!!". The young boy ran over to his mum, who shoved him onto the bench while grabbing the smaller infant she had by her side in a pushchair as he had began to cry. She then proceeded to curse at all three children and then check her phone and type something on it.

I know it wasn't only me who was uncomfortable in the train car - there was silence all around and people were trying their hardest to pretend to look elsewhere.
Granted, I know that being a mum is very stressful, especially with 3 kids below the age of 8, but it really didn't sit right with me.

Should I have said something? I mean I would if she had been physically abusing the children in public of course, but this was nothing like that. It just wasn't very nice.

What do you do, just walk away and hope that the mother was just having a bad morning and that the kids really are loved and cherished? Ugh - this is going to continue bothering me for a while.