Hallo, I would like to order a return train ticket from Beijing to Zhangjiajie. Can I order it on internet?

Yes, of course and I think you can use this website to book. The service is available in "Train" Section on this site.
The train leaves from BJ Railway Station.

Thank you for your answer, I have already tried it several weeks ago, I filled up everything, all required data, but when I finished, I didn´t get any order number, or details for paying. Somebody said, that possibility to book trains is only 20 days before and then somebody else said, it is not possible anyway. So I will try it again, thank you for help