When planning our train travels through Eastern Europe, we had come across several warnings about Polish overnight trains. What had me most nervous were 'gassing incidents' - passengers in coupes who were gassed before being robbed of their belongings. We checked with several sources and everyone said it was safe now and that the incidents were more of an urban myth.
We got safely (and uneventfully) from Prague to Warsaw. Interestingly, we were told at the ticket counter that the only available tickets were 2 upper berths in a 4 passenger couchette. We bought it grudgingly with visions of being pushed off the upper berth by my toddler who I would have to share it with.
The coupe turned out to be 6 berth couchette and completely empty! So much for the Polish train reservation system!