It's also possible to travel in 1st class (2-bed-compartment or 1-bed-compartment).
It's of course more expensive than 2nd class:
The Citystar-ticket costs 50% more in 1st class.
Reservation prices for "Spalnyj Vagon" (or "Double, 1/2" as known by the reservation systems) are about twice as high as for Kupe (T4, 2/4).
You can also book a spalnyj-vagon for single use, meaning that you are alone in a 2-bed-compartment. This (Single, 1/1) costs about 4 times the price of a T4-reservation.