Tibet train from Xian or Xining?

Mr Martin,

Thank you for the reply. We hope to catch the overnight train from Xian on Aug 5, but was told by a couple of Tibet tour agents that it will be harder to buy tickets from Xian but advised to go from Xining. What do you think? When should we send them the documents for TTP? Which train should I catch for best timing of scenery in Tibet?