Reversible seats. My favorite: New Orleans streetcar. 8-second video.



Japanese train version:



Public bench version:




Previously:

Nesting chairs - GIF
Human Hoist collapsible garage work chair
Automatic rising deck chair - GIF
"Chairless chair" wearable ergonomic mechatronic exoskeleton chair - GIF and video
table saw jig for scooping out chair seat