OK thanks guys.
So I will buy my train fare as soon as its available on Trip for Shanghai to Beijing as soon as its available. As there will be two of us this time, will I be able to chose seats to be together? I booked this journey last year and I'm sure the only option was window or aisle. We obviously want two seats together.
For Beijing to Xi'an, I will wait until 2 weeks before and make a decision on Train or plane. Maybe even treat ourselves with Business on the train! Does anyone know which trains have the kiosk style cabins? (I've a feeling that may be on Shanghai to Beijing only?)
Xi'an to Shenzhen is all done now, so lessen learned. (I did have a quick search before posting, but alas obviously too quick!)