Here, naturally.

You have several options.

1. Platskartny is one. The condition of RZD rolling stock varies enormously, so you may end up in a brand new car or one with holes in windows and a filthy bathroom. The quality of linen may also be not so good, I think platskart linens are hand-me downs from CB and coupe. Also keep in mind that platskart cars have 9 4-berth open compartments with berths positioned across (numbered 1-36, odd numbers for lower berths) and pairs of narrower berths running along the opposite side (No's 37-54) where the lower berth converts into 2 seats and a table (which means that you don't get to go to bed until your top-bunk compadre is ready to retire) Seats at both ends of the car are less desirable due to the proximity of toilets. There's very little privacy in these cars and you may get some riff-raff with questionable manners.
   
2. On most sleeper trains they also now designate a 2nd class (and a 1st class) car for separate M/F coupe compartments (and I suspect that would be the "chick" car of the train )
   
3. There are also 3 high-speed trains with open coach-type seating. No's 159/160 The Avrora, 165/166 The Nevsky Express, and a weekly 167/168 ER-200, USSR's original high-speed train. These are slightly more expensive, running RUR 2500-3500 (for 3500 you can get a 1st class sleeper). There's also train 23/24 which offers coach seating for a price comparable to platskart. (around RUR 500)

I've traveled in 2nd class on the Nikolaevsky Express (No 5/6) and found the interiors a bit tired and dusty. On the way back it was 2nd class on a 'no-name' train, and the car was new, bright and airy.

So lots of choices and little consistency. Personally I think travelling on an overnight sleeper in 2nd or 1st class, drinking "chai" and making new friends is a more authentic experience and I would go for the tried and true Red Arrow. You can get a lower bunk in their M/F coupe car for RUR 2100+, but that would be a specific request that should be preferably booked in advance.