Russian Railways is offering discounted upper-berth tickets on the double-decker train #35/36 from St. Petersburg to Adler. The promotion runs until February 25, according to Travel.ru.
A full-trip ticket from St. Petersburg to Rostov-on-Don and Krasnodar costs 2,999 rubles, to Ryazan 1,999 rubles, and to Voronezh (or from Voronezh to Adler) 2,399 rubles. It's worth noting that train #35/36 typically runs every other day in winter and spring.
Tickets are on sale until February 25 for flights through April 24 (except for departures on March 22 from St. Petersburg and March 24 from Adler). The promotion also applies to shorter routes on this route.
Source: travel.ru