Is it possible to buy a ticket directly in the train station if I do not speak any Chinese?

I mean, do they have service in English at the ticket windows in the train stations? I am especially interested in the train stations of Beijing, Xi'an and Shanghai. By the way, how long in advance should I buy a ticket from Beijing to Xi'an if I plan to travel in the middle of September? Is September 17 a rush date? And how about Xi'an to Shanghai in September 21?
Thank you very much!