Dear Peter,
Thanks for contacting Top China Travel!
There are 31 kilometers from Xining Airpot to Xining Train Station, it takes at least 40 mints to get the train station by taxi if there is no any heavy traffic jam. Therefore it might be a little compact for you to transfer from the Xining Airport to Train Station. And if your flight is delayed it will definitely influence you to catch up the train in time. So i would suggest you to book the flights in the morning and the train in the afternoon which has several hours' spare time so that you have enough time to catch the train even the flight is delayed for several hours.
Any further request or question, please feel free to contact with us.