transportation from Chengdu to Chongqing

Any suggestions on the best way from CD to Chongqing ? We will reach CD in mid-Sept & we plan to go to Chongqing for a few days & then return to CD.

Is it possible to buy the train tickets once i reached CD railway station in Sept ? Or to buy the train tickets in advance? Is there a website where i can buy flight tickets or train(fast trains) tickets online ?

Thanks for sharing.