I thought of putting a sticker on my roomette door, but re-considered because it would indicate to others that the room was unoccupied. (You can’t lock your door from the outside.)
Rather, (besides remembering the room number), count the number of rooms you pass until the end of the car. As to which side of the aisle, take note of which side your room is on (L or R) when you are facing your destination. It will, of course, be on the opposite side when you return.
As to identifying which sleeping car, there won’t be that many sleeping cars on the train, so you just have to count how many you pass through on your way to whatever car you’re going to.
Tracking which way to walk when returning to your sleeping car is fairly easy. When you leave your room, note whether you’re walking in the same direction the train is traveling or against it. Head in the opposite direction when returning. (Of course, if the train isn't moving, this idea is not very helpful

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By all means, take the roomette instead of coach – especially on the overnight trains. I’ve traveled single in the roomette several times, and it is perfect for comfort and solitude when you want it. And your sleeping car attendant should be quite helpful explaining the basics.