This site is best for current info: seat61.com, but read full story, as the updates come at the end.
The train LT->LV has been cancelled since covid outbreak and not returned.
From PL to LT it is weekends only tiny diesel by {PolRegio-SUMMERtime only-said to end 1st week sept. Bialystok-Suwalki-Kauno. FRI/Sat TO LT, Sat+sun-mornings- FROM LT to PL.
You will loose many hours by doing this train shuffle compared to the many direct buses/autokary Warszawa-Vilnius. There was also a recent long thread about how to use pass in Lietuva=Lithuaina=LT/Littauen. Find and read it to be informed. Many passholders tend to be a little bit stubborn in using trains only-in some areas this will simply just be an annoyance (there are some special forums auf deutsch-die für jene Bahnverrückte alles erklären und gleichgesinnte besorgen)