The Caledonian sleeper is hell on earth. I did it once. The only worse experience I had was a rotten old YHA hostel in the 90s where I stayed in a bunk room with three people with bowel disorders. And it wasn’t much worse.
I only used it twice, and it was fine. Only fine -- it's a train, not a 5 star hotel. But the bed was comfy enough, I slept OK, and the scenery in the morning was really nice.