I'm going to look into going directly form airport to Beijing hostel instead of airport hotel. Then we will be able to spend 3 days in Beijing.
Do not bother with an airport hotel. PEK airport is relatively close to central Beijing, connected by express railway/subway. It's also a much more pleasant place to spend time.
I suggested a night train to my Dad before from Beijing to Xi'an but he thinks it will be better to go during the day so that daylights allows us to see what we are passing by. At this point though everything is up for change.
The train ride from Beijing to Xi'an is not terribly interesting. Very polluted flat plains. There are some hills but nothing to really keep you interested. You wont get really interesting train journeys until you get out to Xining.
