This is the 17011th most frequent Russian word.
спальных
The word "спальных" is the genitive plural or accusative plural form of "спальный," meaning "sleeping" or "bed" (as in "bedroom" or "sleeping areas").
In this sentence, "спальных" functions as an adjective describing "районов" (areas). It conveys that these are areas primarily used for living.
Москва имеет много спальных районов.
Moscow has many residential areas.
Here, "спальных" describes "мест" (places), indicating that these are designated for sleeping.
В нашей гостинице есть десять отдельных спальных мест.
Our hotel has ten separate sleeping places.
In this context, "спальных" modifies "вагонов" (cabins), specifying that these are cabins designed for sleeping accommodations.