Originally Posted by PhilMHG

that's no problem at all. You can get the ticket directly from one of the ticket kiosks at the train station for any train on the whole G-route network. That's what I did in Shanghai: Bought the ticket to Nanjing as well as the return ticket from Nanjing to Shanghai. You will even get a message that tells you that you just bought a ticket which you cannot board from your current location.