I think its just PTSD from being rejected on the train...... I think
Actually it's ptsd from being molested by his professor on the train, and the school believing his attacker instead of him. The professor is still teaching like he did nothing, and the main character switched schools for his graduate degree to get away from him. Distance didn't stop the panic attacks though, and now he has this new student who came from his old school bugging him too.
I didn't realize he was molested on the train. I thought it was that he'd been dating the professor, but when the professor got caught and lied and said that homophobic stuff, it made him anxious and then he got on a train and had a panic attack and now being on the train is a reminder of that and induces panic attacks.
... I'm wrong? ^_^; I mean, I considered molestation, but I didn't see any flashbacks like that and they didn't say anything about the professor sexually harassing students, so... but if I'm wrong, I'm wrong.
Well it wasnt fully revealed so i guess we're all guessing.....
What is his phobia