My personal favorite is train when travelling between the cities, it is so much nice if you are not in a hurry.
because the landscape is changing a lots in China, and you get a chance to see a real china.

However, there are number of things you need to know,
never get a slow train, it stops every tiny station
get one with air condition in the summer
get a first class ticket so you have own spaces, better services.
food on the train is not great and expensive, so bring enough snack if you going on a long trip.