I suggest you buy tickets online and by the way, it is only about 4 hours by express bus between Zhangjiajie and Changsha. I prefer to suggest you travel by bus. Trains for trips of Xian-Beijing, Xian - Shanghai, etc, also have the same problem. If in public holiday, you should book the ticket 10 days in advance, even no tickets left during the Spring Festival. If not in those peak times, you should buy train ticket 3-5 days in advance, or there is no sleepers left. Not possible to get a seat on same day of leaving. So~