what do you mean "the main train line" - Train operator???
The national and biggest train operator is PKP. Regional networks are funded and contracted out by regional governments and may be operated by PKP or another company but generally work together. There are some long distance trains, especially to Czech Republic operated by private companies with less integration.
for your plans try https://www.intercity.pl/en/
never go near Rail Ninja. And if you ever find a website in english with a name in the format of "country Trains", scroll right down to the bottom and see if it mentions Rail Ninja. Yes Poland Trains does exist but if you look closely the pictures are poor quality AI.