A.
I went through this with ds (5) and now our new baby. We found with ds that when he turned forward and was in the bigger seat, he was fine. My guess is that he was carsick. He tends to get nauseated when riding backwards on the train still. We used to take the train as much as possible because then he could at least be held for that time. We also went at naptimes on a full belly. I took Tylenol with me everywhere. Once in a while I would give him a dose. I figure I know I get a headache when I cry, he may too...
I have also tried pumping with the new baby and taking a bottle with us. Hasn't seemed to help yet.