I am doing the tutorial, and everything seemed fine until I had to build the metro. For some reason, I can't get it right. I add the underground stations at both locations, then I add the track between them to connect. I then go to add stops, and add the stops. I buy the metro vehicle for the line, and then when i go to activate the line, I get the message it couldn't be activated because it isn't closed. The manual says the metro does not have to be closed. So, I loop the line, and I still get that message. What am I doing wrong?