Sorry, I have to disagree here. Trains do not make multi-meter sudden vertical movements.
At any rate, the concern is about egress in an emergency. In a train, every window is an emergency exit. In a plane everyone has to go down the aisle to the nearest exit. The ladders are in the aisle during use and will inhibit that process. They could of course disable the ladders during an emergency so people would have to jump or climb down, but that would certainly lead to injuries and other mishaps that would also inhibit the escape process.
Hard to see the FAA certifying this unless the aisles are made wider. That would reduce overall packing density, so there goes airline's interest in the approach.
It is beautiful in concept. Who wouldn't want to get horizontal during that redeye back to the east coast or the long haul to Japan? Its just hard to see the profitable and safe path to adoption.
