I will be travelling from Shanghai to Zhangjiagang at 20 Oct but don't know the transportations

I'm afraid you cannot travel by riding the train to Zhangjiagang. Actually, the train only operates between Pudong Airport and Longyang Road, non-stop on the way. I have no idea of your exact location in SH, so it is a little hard to tell if it is worthy to take the train. Anyway, no regular train service is available from SH to Zhangjiagang, so I would suggest you go to local General Long-distance Bus Station (near SH Railway Station) to your destination. This should be the best choice.