Keep in mind that the only people who will have access to the sleeping car are other people who have paid many hundreds of dollars (or many thousands of AGR points) to be in a sleeping compartment. They aren't usually the type of people to swipe a suitcase.

Typically, during station stops, the car attendant will be standing by the door, so in the absurdly rare chance that someone tries to jump on and attempt to steal a suitcase, they'll be noticed.

I have taken many long-distance train rides and have had no issues whatsoever with leaving luggage in the lower luggage racks--even in coach. I wouldn't worry about it at all. Just keep electronics in your room (as everyone else has suggested); your socks and underwear will be fine.