Three nights on the train in a sleeper might be cheaper than airfare plus three nights in a hotel, but in most cases, the alternative would be spending those three extra nights at home, which comes at no additional cost.
As others have noted, choosing to take LD trains vs. planes (especially for those looking at first class fares for either mode) is a matter of preference for how to get there, and not one of finding the cheapest mode (which neither first class air nor sleeper by train qualifies).
You may be correct, if you're talking about the train (or plane) as merely a way to get to the destination, but... I would not be spending the three extra nights at home, because my idea of a vacation is a "hotel" that is moving along the rails and past the views, ha, ha! In my case (and I suspect certain others on this discussion board), we *are* comparing those three nights to three nights in a hotel! BTW, if just "gettng there" is the idea, I will probably be flying (and hating every minute). As they say, "when you fly, the vacation starts when you arrive at the destination, and when you take the train, the vacation starts as soon as you step aboard."