If you've seen old-timey photos of aviation in the early decades of the jet age and wondered where all the glamour went, you've been flying economy. When airlines change the back of the plane, it's usually to pack in more passengers or install something you can give them money to enjoy. For those who turn left as they board, life aloft is swankier than ever.

But last week at the Aircraft Interiors Expo in Hamburg, economy fliers got a bit of good, if not quite glamorous, news. One of the world's largest plane builders, Airbus, and cabin builder Zodiac Aerospace are going to start building beds for hire. Full-size, lay-out-flat beds—no business or first-class ticket required.

In an environment as cramped as a plane cabin, Zodiac's designers had to go downstairs, into the cargo hold, to find extra space. Airlines buying certain planes from Airbus will soon be able to order new “passenger modules,” the exact size and shape of a cargo container. They can slot them into the belly of large planes, just like they load your luggage, in a large metal bin. They can also pull them out again for shorter flights or when they need more cargo space.

Inside, these modules look like a cross between an old fashioned sleeper train and an upscale, minimalist hostel. The surfaces are all finished in glossy white, with subtle lighting along the ceiling and floor. It looks like curtains could be added for privacy. This place isn't for the claustrophobic: There are no windows, and the beds are narrow and stacked two high.

Putting people in the cargo hold (which, yes, is heated) isn’t as crazy or as novel as it sounds. Large airliners like the A330 and A380 already have bunks downstairs, where crew members can rest on long flights. On the A380 super-jumbo, the bunks are stacked three high, and accessed via a steep set of space-saving stairs. Boeing typically tucks its crew beds in a narrow space above the passenger compartment, with a ladder hidden behind what looks like a bathroom door. Pilots often have their own beds too, either in the cockpit or nearby.

These spaces are always a little cramped, but still offer the allure of lying down flat. So it makes sense that Airbus thinks its airline customers will want to make more beds available to more people, for a price. Airbus says they’ll be aimed at economy-class passengers, who would still have to spend takeoff and landing in a regular seat—the sort that's been through extensive crash testing. But during a flight, fliers could rent a bunk, presumably for less than the price of a lie-flat business-class seat, and get some proper rest. On very long flights of 12 hours or more, airlines could rent them for half a flight, change the bedding, and then give someone else a chance.

If the concept works, and airlines find ways to use the bunks to make money without robbing too much cargo space, bunks could be just the beginning. Airbus also showed plans for a lounge, a conference room, a medical suite, and a kids play zone, all to be slotted into the cargo hold. One day, the entire plane could feature swappable modules instead of permanent, regular seats. Airbus’ Silicon Valley outpost, A3, tested hot-swappable cabin module concepts with cafés or spin-class bikes inside. But putting those into production would require extensive modifications to the whole airframe.