There are no showers on the trains. With no stops, that would mean about seven days without shower. If nothing else, it is a motivation for getting off and exploring a city or two. The problem isn't as bad as you might think. For another, there's hot and cold running water in the toilets that, with a little care, allow for a sponge bath.
Attendants clean the car regularly.
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. Some people even add a belt for extra protection.