Hello, Mr. Agusti, the train tickets can be bought from the departure city, thus you need to buy it in Xi'an. Usually, you can buy the tickets one week before you take the train. But if you travel during the peak seasons, say, Chinese National Holiday, Spring Festival or some other festivals, you'd better buy it around 10 days before once your finalize your travel schedule, since the train tickets will be under great demand then. Good luck!

I am afraid not. You can only buy the train tickets at the departure city.