It really almost is. Especially on the California Zephyr[0]. We didn't put train tracks down at random. We built them early and used techniques that saw them follow terrain features that were convenient to follow. I ride quite a bit and the mountain tunnels are the only part I can think of that don't fit my description.
It really never wins against a plane or even car except where everything lines up perfectly and you can treat an overnight sleeper train as a rolling hotel, check in, eat, sleep, eat, walk out where you need to be.
It’s such a good experience that building the rest of the trip around it can be worthwhile, but it is still not terribly fast.
Yeah, if you get a sleeper and figure you're saving a night in London or Edinburgh and you don't need to deal with airports, something like the Caledonian Sleeper isn't a bad deal or really consume a lot of extra time. But if you're not in a hurry, it's still a good experience for a lot of people.
But then I've also done a trans-Atlantic crossing but I was semi-retired at the time. Which I would do again at some point but was certainly neither cost nor time efficient.
The worst part of any scenic train trips are the inner cities (which are interesting and different, at least, and you can learn new swears from the graffiti) and the cornfields in the plains.
Even the latter has things to see as rail lines go through so many small towns. But it’s admittedly not as cool as the mountain or shoreline trains.
Just don’t sit on the bottom floor or you’ll be staring at cornstalks for 30+ hours.
In fairness, "amber waves of grain" can probably get old after a while. But absent time (and, to some degree, money restrictions), I'd probably get somewhat bored after a while given that I've seen a lot of that scenery from a car or hiking around.
But to the point some people here are making, I've done some fairly long distance train travel in Europe, including sleepers, and, while I tend to prefer it to planes especially budget ones which I basically never take, I'm not sure it's an especially efficient way to travel for the most part if you want to get from point A to point B as quickly and cost effectively as possible.