You have been cited under § 2.07 of the Public Courtesy Code: Seat Overexpansion
ONE SEAT. ONE PERSON.
A seat is a defined region with understood borders. You are occupying yours and a meaningful percentage of the two beside it, while the car fills and people stand. Close the stance. It's a train, not a recliner.
This is not a fine. There is no fine. Think about what you did.