It takes around 38hrs from Chengdu to Shanghai by train. So, taking hard seats is really toilsome. And usually it is not feasible for you to stop along the way at the touristy towns and reconnects in the same day or next day using the same ticket. Personally, you would better choose to take berth. For the tickets reservation, you would better arrange it in advance like making online bookings or contact the tickets offices at least 10 days before departure. Good luck!