what is the train station to go to Chengdu from Chongqing? how frequent is the train and how much is the cost?

You should take the train at CQ North Railway Station, and there are more than 30 trains to Chengdu every day. You can see the timetable on the train page of this website.

CRH train ticket fare (CNY):
96.5 second-class seat/116 first-class seat
Normal train ticket fare (CNY):
46.5 hard seat/73 hard sleeper/120 soft sleeper