Security on Board
For security in the wagon the prowodnitsa is responsible. When you enter the train she takes your ticket and assignes a bed to you. Wagon and bed number are printed at your ticket but sometimes you will get another bed, so that groups with beds in different compartments can move into one at least. On bord of the train it's very narrow so it doesn't make sense to get on hasty. Keep cool and rest and you'll be welcome.
At stops the prowodnitsa counts the people who go out and in. To avoid confusion and delays you should use the same door when leaving and return to train.
We heard about thefs but never had experienced one by ourself. Make sure that at least one person is in the compartment to look after your things or ask the prowodnitsa to lock your door when you leave. Valuables and cameras don't should be left visible. In general travelling in wagons of the transsib is very secure, not at least because all the way every wagon is serviced by prowodnitsas who know which person belongs to "their" wagon. In case of problems you can talk to them at every time. If necessary they will discuss the problem with the chief of the train. Additional you can find a post of the railway police at every bigger train station.