Do you have to buy tickets in advance? Or can you get them on day of travel at the station?
We took this train last month. I bought the tickets online ahead of time and was glad I did. The train station in Warsaw was very busy, long lineups to buy tickets. Mind you, it was a Friday and right before a major Polish holiday. I guess it would depend on when you are travelling. But I prefer to buy tickets ahead of time.
We took this train from Krakow to Warsaw in late July. We also bought our tickets online in advance, and we too were glad that we did. The loading platform was crowded and a little chaotic. The train looked full to me, so I'd suggest buying in advance.
I took the train from Gdansk to Krakow in May 2018. I would advise booking ahead. It's not that hard, will give you peace of mind and maybe save you a little money.
Buy in advance. And allow enough time in the station - it's big and I thought it was confusing. I went to the station a day early to buy my ticket and spent half an hour or more in line, only to find out that I was in the wrong line.
That Gdansk to Krakow train ride used to be longer than it is now., a very historical route going through those towns at either end, first through the region of the lower Vistula.