How do I collect advance booking train tickets for Xian - Luoyang & Nanjing, and Nanjing - Shanghai?

May I ask you where do you book the train ticket?
If you book through some tour agency, I think the tou agency will help you to get the ticket.
If you book online on yourself, you need to get the train ticket from the departure railway stations. It is suggested to get the ticket in advance, so that you will not be in a hurry.