They are called platform edge doors and they are common in Asia, and rare in Europe or the Americas. They make the platform a much more pleasant place. Full height walls and doors makes the platform be a normal interior space.
If you've travelled that far you have seen those doors. They are included on every automated people mover at airports. Securing the track area is required for driverless operation, so every automated people mover will have them The doors are seen as optional in most subways and light rail so they are often cut from the project.