I know that some moms are uncomfortable with public nursing, and I am trying to be sympathetic to that, but I have a hard time understanding how forcing a hungry baby to wait is more important than your own comfort level or those of the strangers around you. With my first child, I was a little leary of public nursing, but sometimes it was necessary. I forced myself to step outside my own comfort zone because my son needed me to. It didn't take too long and I was completely comfortable with public nursing. By the time my 2nd son came along, I nursed him in public all over the place and not one person made a comment to me ever.
If you ever find yourself in a situation like that again, maybe just try nursing him instead of waiting. Focus all your attention on his angelic little face and completely forget that there are others around you. If any comments are made, ignore them and let you husband deal with them.
You may also want to consider trying a sling. In your situation on the train, a sling would not only offer completely shielded nursing, but it would've also allowed you to leave the stroller at home and you could've gone up to the upper level and nursed a little more privately.
Your baby isn't going to care how many people are watching him eat. He is only going to care that he gets to eat.