As you may have noted by now-here in EUR all stations have counters-often even quite large, where people work who do their job-selling normal tickets-and also all the permutations that belong to that-like ONLY RES when ticket is held already. In such large towns they will be extremely very well used to such questions.
You have also been told by now a few times that a planner like that of bahn.com will show all-also what you request. Just open it and play a few mins with it to get some experience.
You CAN get to Lietuva=Lithuania/LT by train-but not direct from WAW and only weekends-and have to come back same way, the total time from WAW is also much longer as by direct bus-so for most its 2nd choice.
NOTE: in PL you canNOT use any train with the pass-in/around WAW are green trains of KM=Mazowieckie for local trips and IN town only red/yellow (=the city colors, just like the trams/bsues) SKM-these do NOT take passes. IN PL the long distance trains are run by PKP (as TLK, IC or EIP) and these must all be REServed (in theory). For free with pass-except EIP (fast pendolino).
Next stop: Krakow, far more interesting as this city with all its skyscrapers-old & new. And its tiny completely rebuilt to look as old ´old town´.
I was in Poland last week and made my reservations for the EIP Krakow-Warsaw and EC Warsaw-Berlin at the ticket office in Krakow Station, worked well. Reservation Krakow-Warsaw was only made on day of travel and there were plenty of seats left, our coach was only about 20% occupied.
For countries were reservations cannot be made online, I typically make them at the ticket office of the respective country. To ensure we don’t end up with full trains, I regularly check the occupancy for the train I want online through their regular sales channel. Most European operators have online sales but only a few have online reservations (for Interrail). This does not work for France as they’ve quota.
EIP Krakow<->Warsaw was a nice experience, modern train and complimentary light dish+drinks in first class.
Part of our route last week was Ljubljana->Zagreb->Budapest->Vienna->Krakow-Warsaw->Berlin. We had no issues and all trains were in pretty low demand.