It probably sounded like a civilized solution to the increasingly congested New York City of the 19th century: to ease crowded streets, “moving sidewalks” or “moving platforms” would be built underground.
The idea was first proposed in 1871, then more seriously in 1902 for the Brooklyn Bridge.
Widely debated in newspapers at the time, it went no where: Mayor Seth Low killed the project.
But it popped back up again around 1910, this time as a network of moving sidewalks at a top speed of about 10 miles per hour that would replace the new subway system.
So why didn’t the idea fly? Perhaps the subway companies had too much political clout to let it happen. Or maybe subterranean roller coaster cars didn’t move people as efficiently as a subway car could.
In the end, the idea kind of lives on—inside city airport terminals.