just a note...

about trains in China... to/from Lhasa...
(don't have the experience to comment on other destinations...)

tickets...
you can only buy tickets with 10/7days in advance...
you can only buy tickets in the station where you will take the train... or by yourself or by using a travel agency to go there on your behalf...
there is a limited number of soft class & hard class sleepers... and millions of chinese travel by train...

sleeping...
Hard sleeper (open 6 plan bunks) - Upper bed (cheapest... safest... hardest to climb... impossible to be in seat position...), Middle bed (middle price... safe... not hard do climb... impossible to be in seat position... but higher space...), Lower bed (the most expensive... but the only one that allows to seat on it... since is accessible... if you don't pay attention... everyone will seat and even sleep on it... because here a person doesn't ask first... a person does...)

bed sheets...
are only changed before the train departure... and after the train arrival... which means that if you took the train somewhere in between... you're not the first...

food...
this route luckily is used often by westerns... so if your destination is Lhasa there will be in the restaurant car english menus available and more dishes close to western taste... vegetables and fruit are always a saver... and in every train stop you will find street vendors with other food choices... also over the day the train staff will route thru the corridores selling drinks and snacks... but better if you travel with your own food... specially if your not in a tourist route or if you are already leaving Lhasa... because in these cases english doesn't exist and the menu falls somewhere between chicken feet or bird stew... :P ...