The title of the post may sound absurd, but with the right kind of promotion, a late night train could possibly be an attraction for travelers on a route that is less than 750 miles long. Amtrak used to have sleepers on Trains #66 and #67 when they were known as the Night Owl.
I'm looking to the Rockies and Florida specifically. The Denver-Salt Lake City route is too hilly to even be considered for high speed service. However, another long-distance route and a couple of regional routes should aid the existing California Zephyr. Meanwhile, Florida East Coast Railway could have a limited stop overnight roundtrip. Back in the 1950s, the railroad operated several nighttime trains between Jacksonville and Miami. A third likely candidate is the Coast Route between Los Angeles and San Francisco. Southern Pacific ran a bunch of routes between the two cities during the Golden Age. Even though it wouldn't match up to the planned HSR route via the Central Valley, travelers would have an alternative that would allow them to take it easy.
Over in the UK, First Group operates overnight sleepers via their First Great Western and Scotrail subsidiaries. On Scotrail's route, First Class riders get a single berth while Standard Class (the Brits' name for coach) get a double berth--bunk beds shared with other passengers. At 75 mph, the trains aren't high speed like other First Group's routes, but travelers aren't riding for speed. In due time, Express HSR operators will likely follow China's model and operate overnight sleeper service.
First Group makes sleepers affordable, and with a marketing genius, its American counterpart or another operator can also provide the same level of service here. There would have to be some tweaking. First, the train's First Class component would have to include extra amenities for it to stand out. Second, a new name would have be to complied for the bulk of the passengers who wouldn't be traveling via First Class. Finally, the fares would either resemble today's Business Class fares or a hybrid between Business and Coach classes.