From Chongqing to Shenzhen, you need to take train K775/K778 from Chongqing north station to Guangzhou first with the train schedule of 16:18 to 14:11(the next day). Then take a bullet train D7007 15:23/16:41 from Guangzhou to Shenzhen. K588/K585 also can take you from Chongqing to Shenzhen directly, but this train starts from Chengdu and passes by Chongqing in midnight. It is hard to purchase the tickets in Chongqing. You can have a try.

From Shenzhen to Chongqing, you can take K586/K587 at 10:40 at Shenzhen west station and arrive in Chongqing north at 18:13 of the next day.