I would and have intervened when people (particularly women and children) or dogs are being threatened. Even though once I got an ear bashing off a girl who was being dragged around and and shouted at by her boyfriend and I stepped up (no one else would) and asked if she was okay. I guess she was enjoying it in her own sad way.
Sometimes no one has come to my aid when I've been pestered on public transport, flashed at etc, but I do remember one young guy stopping to help when I was being harrassed by a group of school boys.
Once I took my parents, my niece and young toddler on a day out. We were coming home on a train and my father had to sit on the other set of seats across the aisle and promptly fell asleep. More passangers got on at other stops including a young man who sat opposite my father. As we were approaching our stop I began to open my buggy, sort out my bags etc. My father had woken up and as is the humour of our family, began to berate me, and all mothers, for cluttering up trains with their big pushchairs, in his deadpan Glaswegian. The young man imediately begun to defend me and told my father he was completely unreasonable - before I had a chance to explain that he was actually with us and joking. The young man was pretty embarrassed but I told him that was a really nice thing to do.