Yes, you can buy it at the train agency.
Just tell the conductor the date when you want to return to Jinan from Chongqing and buy the train ticket for that day. No trouble! If you plan to stay in Chongqing for n days, you can buy it seven days in advance. Moreover, there must be many people taking the train during this period, and they will arrive in Tomb-Sweeping Day soon.
Handling fee, 5 yuan!