Hi, i'm going to Changsha then Zhangjiajie from Hong Kong by train.

Well, there is no direct train to Changsha from Hong Kong. You are advised to travel by air (about 1.5 hours).
Then, you may take an ordinary train to ZJJ (about 5.5 hours) for no high speed train is available.

Or, you may take a bus to ZJJ from Changzhutan Bus Station which is near to Changsha Railway Station. The buses are available from 8:20 to 19:00, and the trip takes about 4.5 hours. The fare is about CNY 120 per person.