Since January 4th 2013, the selling periods of Chinese train tickets are 20 days before the departure day for bookings online and by telephone, and 18 days in advance for train ticket windows at train stations and local train ticket offices.
Chinese train tickets are often sold out, especially during holidays such as weekends, summer vacations, Chinese New Year, and Labor Day and National Day holidays. Therefore book a train ticket in advance as early as you can. However we cannot always guarantee the availability of the ticket you want at the time of release (usually late afternoon), due to the masses of people buying tickets.
Passports are needed when buying a Chinese train ticket and boarding a train.