Just think about all the motion involved and you can get an idea of the problem. Apart from the toilets, no one should have any cleanliness complaints. Attendants clean the car regularly. There are two classes of travel on the long distance trains. The only difference is that first class has two people to a compartment, second, four.
Second class provides more opportunity for interaction with other travelers.
It's hard to be in a compartment for a couple of days without some type of communication taking place. Security Common sense is your best protection. Keep anything you don't want to loose on your person at all times. There are lockers under the seats and you can have the attendent lock your compartment when it is unoccupied. You can also lock the door at two points from the inside.