Available tickets of soft sleeper (4 Nos.) from LHASA TO BEIJING in the month of July or August 2011

We are from Delhi, India and want to travel in china form Lhasa to Beijing in holidays by train. Further want to know the availability of tickets in the month of July of August 2011 for Soft Sleeper (4 Nos.). Also the procedure of getting online reservation and tickets. Awaiting your prompt response. Thanks & Regards.... Madhulika, Delhi