Route inquiry and how to buy train tickets

Hallo,

we're going to visit China next month, once we leave beijing we'll wanna visit Fenghuang, Dehang, and Chengyang... we thought about the possibility of take a night train (Z) from Beijing to changsha, and than go to Zhangjiajie (spend one night) and then take a bus tu Fenghuang. Is this a logical route?. How and when can we buy the train tickets?

xie-xie

Mr. Miquel Martínez