Travel / Optimal order of Warsaw-Gdansk-Krakow for train travel September [8]

Visiting Poland in September 2016 visiting Warsaw Gdansk and Krakow with sidetrips from each. We fly into and back out of Warsaw. We want to use train for intercity

connections. Any suggestions as to the proper ordering of the cities. Especially interested if iut is smart to do the travel between Gdansk and Krakow as an overnight. Is travel one way between those two cities easier than travelling in the opposite direction? Thank!