She isn't back in my town. I looked it up, apparently this is what could happen with Katie: When Katie is in your town, talk to her and she will ask you to take her to another town. Any town will do. Ask one of your real-world friends to open his or her gate so you can travel to that town. Katie will follow you. When you get on the train, Katie will either come with you, or she will drop her ticket and get off the train, but the doors will close behind her and you'll go to your friend's town by yourself. When you go back to your own town, Katie will not be there.