Dziekuje ad mentioning this. Still the tiny 2-car PolRegio dieselset, I assume. All the usual railforums will  burst out in big cheers and profetise the imminent revival of the iron monsters on rails.

Now we have to wait for new reports on how it all works out with the pass in Lietuva=Lithuania.

Apparently, the route will soon be discontinued. According to PKP, this connection will only run between Kaunas and Trakiszki from the timetable change on 3 and 4 September. At least that's what it says in the digital timetable at: pkp.pl/en/
That would be a pity, of course - let's hope that this is just a mistake on the website...

It is NOT run by PKP but by PolRegio.

There wre some plans that PKP (in PL the long-distance train operator) would also open a route to there, going further into PL-likely Warszawa. So perhaps this may even be a sign of more happier things to come.

PR=PolRegio (which runs the local regional trains in most provinces of PL-not all) does not have its own planner, but links into a few general PL ones, like e-podroznik.

I used this connection on July 23, 2022. The train only runs in the weekend.

You can’t reserve seats on this train and your Interrail pass is valid. This train does not have a 1st class or restaurant car. The seats are not the most comfortable to spend a few hours on, but it gets you there ;-)

This train runs (in the weekends) until the end of the timetable. From 11 December, there will be a daily connection. Timings still not entirely final:
https://www.seat61.com/trains-and-routes/warsaw-to-vilnius-by-train.htm