Purchasing train / air tickets between 2 cities in China at a 3rd city, possible?

We wish to visit China on Aug/Sep 2011. Transfer between cities both by train/ flight. I assume this is high season in China. Thus we have to by tickets in advance. (How many days approximately?). Is it possible to buy, for example, a train ticket: BEIJING >>DATONG from the city of Turpan/Kashgar (just an example). Or to by train ticket BAOTOU >>ORDOS from the city of Lanzhou? Or to by air ticket LIJIANG >> KUNMING from the city of Xian? These are only examples of tentative route for our China tour. Is there a difference whether the city in which we wish to purchase our train/air tickets (NOT the departure & destination) is a BIG city? Can we buy the tickets at a small (China dimensions)city - like Turpan , Dunhuang, Dali? By the way, is there a difference with respect to my inquiry between train to air tickets?
Much Obliged
Eli(from Israel)