I may be in the minority here, but I would not leave a laptop in a roomette while I was out to dinner or walking around,etc. On several of my trips with Amtrak, the sleeping car attendant vanishes after we depart and we never see him/her again, so there is often no Amtrak employee keeping an eye on the car. And it is definitely possible on some trains for coach passengers to wander into the sleeper car, I have seen it happen.
Whenever I travel, I always take a very small bag to put my valuables in such as cell-phone, wallet, personal electronics, etc., and carry it with me whenever I leave my roomette.
I have often seen people on the sleeper car leave valuables right out in the open when they leave.
On many trips I have seen women leave their purses/pocketbooks right on the seat!
Yes, you might be OK with hiding the laptop somewhere in the roomette. But why on earth would you want to take a chance?
I have also ridden in coach a few times, and people were always getting things stolen.
Theft can and does happen aboard Amtrak.
I wouldn't assume that just because you are in a roomette you are immune from theft.
Remember, the roometttes do not lock from the outside.
Maybe I am just overly cautious about these things....
Bill