the way from Chengdu to Jiuzhaigou

I'm afraid there is no train between Chengdu and Jiuzhaigou. From Chengdu to Jiuzhaigou, you can take a plane or a bus.
By air: Jiuhuang Airport is about 88 kilometers from the mouth of Jiuzhaigou valley. You can fly to juzhaigou in about 40 minutes. Upon getting off, a half hours' ride can take you to the scenic area.
By bus: Regular buses from Chengdu Chadianzi and Xinnanmen coach stations are in service around 7:00, 8:00 in the morning. It is suggested to buy the bus ticket in advance. You can go to Jiuzhaigou from Chengdu in about 11h.

I am sorry there is no trains from Chengdu to Jiuzhaigou, you can go to Jiuzhaigou by bus or airplane. If you want to take bus, you can go to Chengdu New South Gate Bus Station to take the bus for Jiuzhaigou.