There are many trains between Moscow and St. Pete's, the level of comfort, speed, people, etc depends how much you paid for the ticket.
High numbers of a train (triple-digit) indicate that it is a slow and old train, low numbers (single-digit) indicate that it is a fast and modern train. The crowd in third class (platzkartny) is significantly different from the crowd in second class (spalny wagon).
I don't think you'll have too many problems on your train trip from Moscow to St. Petes, even the raucious drunks are usually pretty friendly. I travelled between these cities in 3rd class on a double-digit train and had no problems. All you have to do is find a safe place for your money and passport. I had one of the lower bunks (I asked for "dobro mesto" ie a "good place" when I bought my ticket) and I could stow all my things in the box underneath the bunk. One could not get at my things without me getting up from the bunk, so my stuff was very safe.
http://seat61.com/Russia-trains.htm
http://waytorussia.net/Transport/Domestic/Train.html
[ Edit: Edited on 04-Apr-2009, at 13:28 by t_maia ]