The train K1002/K1003 will depart from Xian at 20:22 and arrive in Chongqing at 08:58 of the next day. It is a good choice.

I've been to Chongqing for many times from Xian, but only the train 2334/2335 was available at that time. It was not a good experience since there was no air-condition in all the compartments except the soft berth ones. So if taking it in Summer, it will be too hot if you are not in soft berth compartment. Hope it helps.