Ack! The sound of that punch was crazy! He totally broke her nose
I've been in the middle of fights - one was between my dad and uncle and they both outweighed be by about 100 lbs each and stood a good 10 inches taller...I really didn't give a shit if I got smacked by doing it.
I think it really depends on the situation.
But yes, that was disturbing regardless. The police can't get down into the subway that fast - but some cities have special constables hired by the public transit to handle stuff like this. In Toronto, we have emergency bars in every car and you just have to press them for the subway driver to stop the whole train and come investigate the problem.
As for a guy smacking a woman - some are capable of anything and just don't give a crap in the heat of the moment. I had one guy and his friend come into where I work this weekend and the friend was blasting the other for hitting his girlfriend...he didn't look too concerned, even as they were buying first aid stuff for her.