Hello, my family is visiting Shanghai, Chengdu, Xian and Beijing in Dec 2009. We will take a train from Shanghai to Chengdue and then another train from Chengdu to Xian. What really concerns me is that we would like to take a train as well from Xian to Beijing but only planned to stay in Xian for a couple of days, and we want to make sure that we can get sleeper tickets. Is it possible and how?