The Overnight Train Is Back, and It Is Not Nostalgia

For twenty years the sleeper was in managed decline. Routes closed, carriages aged out and were not replaced, and the economics looked settled. A short-haul flight cost less and took two hours instead of twelve.
That has reversed, and faster than most operators expected. New routes have opened across Europe, several closed ones have been restored, and there is new rolling stock being built for the first time in decades.
Why now
Three things at once. Flying became less pleasant and less cheap. Emissions moved from an abstraction to something travellers and, more importantly, corporate travel policies actually account for. And a night on a train removes a hotel night from the trip, which changes the comparison entirely.
That last point is the one operators underestimated. Compare a sleeper against a flight and the sleeper looks expensive. Compare it against a flight plus a hotel plus two airport transfers plus a day lost at each end, and it stops looking expensive.
What has actually improved
The new stock is a genuine step up. Private compartments with a proper bed and an en-suite exist on several routes now, which is a different product from a couchette shared with five strangers.
The old sleeper experience asked you to tolerate discomfort for romance. The new one does not require the trade.
What has not
Booking. It remains close to indefensible. A journey crossing three operators may need three separate reservations, made on three websites, at three different times, because each one opens its booking window on its own schedule.
Miss the window and the cheap berths are gone. There is no single place to see the whole network and no reliable way to hold a connection. It is the one part of the experience that is genuinely worse than flying, and it is entirely self-inflicted.
Book early, book each leg the day its window opens, and accept that the hardest part of the journey happens months before you board.