I took the train from Xining, Qinghai and went all the way to Lhasa, Tibet for a photo trip several years ago. It turned out to be one of the most amazing trips I have ever taken. Within the 12 hour traveling time (from 6 pm to 6 am the other morning) we traveled through one of the highest altitude plateau in the world (average elevation exceeding 4,500 metres according to Wikipedia) with a breath-taking view outside the window.
Trust me, try & visit this place in your lifetime 
you won’t regret it. Not to mention the exciting sceneries when you arrive at your destination: Lhasa!
