https://ru.wikipedia.org/wiki/%D0%A1%D0%BF%D0%B0%D0%BB%D1%8C...
https://www.ozon.travel/help/railway/preparation/trains/
As far as I understand, М has private bathroom but СВ doesn't. It's confusing but I'm sure I saw them sold at different prices.
There were no bathtubs on the trains in 1980-s in Soviet Union...
https://ru.wikipedia.org/wiki/%D0%A1%D0%BF%D0%B0%D0%BB%D1%8C...