If he said this, then it was an act of sexism.
Would this make you feel uncomfortable working with him, if you were pregnant yourself.
Although it wasn't at work, if it makes you feel like you would be treated badly or unfairly if you were pregnant then surely this is something you could raise as a concern at work with HR.
Its not a formal complaint as such, but if you did feel that his attitudes would potentially have an impact on staff or the business then I think you could say something. Its not the fact that he didn't move that's the issue, its his comment about the woman not getting on the train that's highly questionable together with him being abusive towards her. He was within his rights not to move, but he has to explain that politely, respectfully and perhaps help her in another way (maybe ask if there was anyone else who could give up their seat for example).
The type of work you do also makes a difference. If you meet clients or people from outside your organisation its also important, even if you don't wear a uniform. You still could be recognised by someone (after all YOU recognised him) and his behaviour could be viewed as offensive (after all YOU have been offended by is behaviour).
If you saw the same man making racist comments in public though not in the capacity of working for the company, I think there would be less of a grey area here. I also think the fact you are in his management chain also adds another dimension as then technically I would have thought you have a duty of care to other members of staff and to ensure that he is aware of what is appropriate behaviour. The fact it is not in work hours is a bit of a red herring in that respect.
Companies have started taking action against staff who post things on social media about their private life because they can be recognised and this reflects on the business. The social media provides the 'witness' or proof without someone being there. If anything the fact that you were there as an eye witness makes it worse because he is mixing in an area where other members of staff might be affected directly by his behaviour. The only thing you haven't got is proof - hence why it can't be dealt with as formerly as a comment on Facebook might be. But it does show that the boundaries about how you behave in your workplace and private life are not exclusive and you can not just use the excuse that it was outside work.
I don't think its about him getting his comeuppance. I think its about what you consider your responsibilities to be and whether you think his behaviour was enough that it could have consequences.
Its a difficult one, but I do against the grain a bit here in saying I do think there might be a good case to raise the issue as an equal opportunities awareness thing more than you think. If not against him directly, then perhaps to raise the subject more generally as an issue for the company about how staff behave outside of work and how they may be accountable for it even if they don't think they are.