The distance of Xining to Lhasa train is 1972km/1233 miles, which takes 22 hours. It's an ideal place to start your Tibet train tour, as you will ascend your altitude from Xining(2,261 meters) to Lhasa(3,650 meters) gradually therefore you will have enough time to acclimatize the elevation. Xining is the start point of the Qinghai-Tibet Railway and the capital city of Qinghai Province.
The Z6801 is the only train from Xining to Lhasa. It departs from Xining at 14:01 from Xining Train Station every day and arrives in Lhasa at 11:20 on the next day. And the rest are all through trains. The train running from Xining is pressurized, supplied with artificial oxygen, and sealed. The windows have ultraviolet filters to protect passengers from UV radiation.
Besides Z6801, you also have other choices. For better scenery, you are recommended to take Z164 as it passes through the major scenic spots in the daytime.