In defense of another?

I was riding the train on my way to work and all of the suddenly I hear a man arguing with the woman the woman happens to be hispanic and did not know how to speak english well enough defend hersefl. That man happens to be black this man was totally disrespecfull to that young woman and racist you can clearly hear his racist remark he made toward that woman and everything started just because her child accidentaly bump into him accidentaly touch him. I'am Hispanic I work as a security guard and I go to college for a carreer in the medical field was growing annoyed by that man. That man had no idea that she was not alone taking into account all that he said regarding her and her background. I could have easily turn it around and humiliated him ( I guess his mother did not teach him right ). Not only that no one rose to her defense I guess I neglected too. But I would have acted if he had threaten her & son safety.
What would you have done?

Like you I would have waited until her safety or the safety of her child was threatened. It's unfortunate that some people seem to feel that they can treat others with such disrespect.

I would have offered my help to the lady, and offered to translate for her if i was able. I would have also let the man know he was making a scene over a such a trivial incident. A child bumping into you is no reason to go off on somebody. I would have also notified any security, police, etc. that were on the train as to the man's behavior. What he was doing could be considered disorderly conduct in some states, especially if he was cussing.

Clearly, you now wish you would have stepped in and stopped the argument. Otherwise you would not be asking this question now.
Yes, you should have been a gentleman, and helped the woman and young child. Maybe, offered to sit with them and make sure that they were safe.

If you know Spanish, why didn't you offer to translate or intervene instead of cowering in hiding?