In most routes, if you remain on a service as a passenger and leave the final station, you will be presented with a 'You have reached the end of the represented area' screen and returned to the main menu. This doesn't happen in SHS until the inevitable happens (see screenshot below after leaving Faversham heading East).
I have a few times moved the camera over some distant areas (collectables) and it gave me that message. It's problematic putting it in. So I don't really have an issue about it not being in Southeastern.