Poland Trains

Here are the instructions from the train guru, Man in Seat 61:

https://www.seat61.com/international-trains/trains-from-Warsaw.htm#Warsaw_to_Krakow

As noted in this link above, you should use https://www.intercity.pl/ for domestic travel in Poland.

Polrail is a third party agency but has good reputation. You may consider using it should you have problems with 3d secure card payments on the intercity website (it seems to be an issue for travellers from outside Europe).

Thank you!