Train from Zhangjiajie to Guilin via Liuzhou

There isn’t direct train from Guilin to Zhangjiajie. There are 3 overnight trains passing Liujiang Train Station (in Liuzhou) but also stopping at Zhangjiajie. Liuzhou is near to Guilin. Therefore, If you do not mind overnight train trip, you could take an afternoon bullet train from Guilin to Liuzhou first. After that, take a taxi to Liujiang Train station to catch the night train to Zhangjiajie. The train will arrive at Zhangjiajie Railway Station next morning. Then it is easy for you arrange a tour to visit Tianmen Mountaion National Forest Park on the way from Zhangjiajie city to Zhangjiajie National Forest Park ( Wulingyuan). 
The problem for this way is the difficulty of buying the train ticket from Liuzhou to Zhangjiajie, especially in high season. We suggest you arrange it through your agency in China. 

Train from Changsha to Guilin
Take the morning high speed train from Guilin to Changsha is also one of your best choice. You will arrive at Changsha South Railway Station around 14:00. Then you could try to see whether there is any bus from the South Railway Station Square to Zhangjiajie. If not, you will need to take taxi from South Railway Station to Changsha West Bus Station. Then take the bus to Zhangjiajie. It will need around 3.5-4 hours by taking bus from Changsha to Zhangjiajie. Or you can also take taxi from Changsha South Railway Station to Changsha Train Station, from which there is a train from Changsha to Zhangjiajie, which also takes about 4-4.5 hours. ..More

Take flight from Zhangjiajie to Guilin
At first, please kindly be advised that there are only a few cities who have flights toZhangjiajie, such as Beijing, Shanghai, Guangzhou, Shenzhen, Xian, ect. And most flights to Zhangjiajie are at late afternoon or early night. The flights to Zhangjiajie will become less in low season.
Nowadays, there is a flight between Guilin and Zhangjiajie in high season ( April to October) almost every day. Therefore, if you would like to take flight to travel to Zhangjiajie from Guilin, it is possible, but it depends on the availability.