Every other day, there is one train runs between Chongqing and Lhasa. Train T222 departs from Chonqging at 20:18 and arrives at Lhasa on the third day at 16:35 which will take 44 hours and 17 minutes.
And train T224 is the train from Lhasa to Chongqing which leaves Lhasa at 10:00 and reaches Chongqing on the third day at 08:25.
Chongqing to Lhasa Train Route Map
*** Train from Chongqing to Lhasa departs not daily, but every other days,
*** Trains depart from Chengdu and Chongqing respectively, for example, Odd days in a month from Chengdu, and Even days from Chongqing. If unfortunately, Chongqing to Lhasa Train does not departs on your date or you miss the train, you may take a fast train to Chengdu which is only 1.5 hours distance, to take train there the other day to Tibet.
*** Do remember to bring and show your personal passport, Tibet Travel Permit and train ticket to the train inspector before boarding. It is also advisable to have a few copies of Tibet Travel Permit since you may be checked many times about the relevant documents during the train ride. Just do as the staff of railway station says.