I fly in and out of Seattle every week. The flight was probably originally scheduled to depart from B but was changed to the S gates when a slot opened up over there. Not an error, just capacity change. I'm confused about the train - it runs in a loop from the main terminal to the end of the B gates to the S gates and back again. I've never had it not stop at the B gates. Was the escalator/elevator blocked to prevent you from going down there?