There is direct sleeping car 782 Zvolen os. st. - Moskva Kievskaia in the train 933. It runs as need may be. What does it mean? In which circumstances it does run or does not run?
Knowing RZD they might include it during holidays or peak season (like skiing time in Tatras), so basically whenever they see that everyday Moscow-Bratislava carriage is full or almost full. As result RZD feels the need to include one of the additional carriages to satisfy the demand.
Usually it doesn't happen that often on international routes, so it is hard to count on additional carriage. But you can always check its availability for the date you want,
http://www.mza.ru/?exp=1 shows the scheme of the train for the particular dates (but count the fact that carriage might be included even few days in advance)
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