CityNerd is a great channel, but his mode graph leaves out sleepers: Of those 14 hours travelled, how many do you actually count as travel time? I'd say subtracting the time you spend leisurely sleeping and eating at the minimum, make that 10 hours, you might also save on hotel check-in and check-out, the additional travel to that hotel, and other small stuff. Four hours travel time are very competitive.

The schedule is more restricted but I doubt many people visit more than one far-away city in a day. HSR sleepers aren't also really a thing, at least I'm not aware of any it's all conventional rail but that doesn't mean that it's some utopian far-out concept. Over here in Europe sleepers aren't high-speed simply because they don't need to be. And/or because our train infrastructure actually sucks and you can't take a sleeper from Helsinki to Lissabon, quite comparable a route to NYC-LA.

One important thing is to make sure that those trains are actually nice: When the Austrians doubled down on sleeper trains they quickly found out that the more expensive tickets actually sold very well and with newer trains they basically got rid of the whole mid-range, it's either a decent compartment with shower and everything or a capsule. Business-class or hostel-class. People are willing to, and almost demand, to spend money on the ticket that they would otherwise spend on a hotel room for a night. Lean into that, make sure the bread rolls are crunchy and the coffee has a decent standard and people are going to flock to it. About all the staff having a Viennese accent of course doesn't hurt the ÖBB.