I am also well over 50, and my last few coast-to-coast trips to see my family were solo. I have never felt insecure in my room. In the daytime, I only lock the roomette door because the motion of the train sometimes causes it to slide open. At night, I always lock it so no one will disturb my sleep, since I know people may make mistakes when they are drowsy. I have never experienced or heard of anyone entering a room with bad intent.
The lock is constructed so it can't fall open or be jimmied. It's very secure. Here's a short video showing how to close and secure the lock:
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As far as I know, this is accurate for all sleeper cars currently in service. It's the only sleeper compartment lock I have seen on Amtrak.
I'd also like to note you cannot lock your compartment from the *outside* when you are away from your room. I just close the curtains, close the door, and leave. No one has ever disturbed anything while I was gone.