It is a Hashtag widely used on Twitter by railway enthusiasts, which refers to trains with carriages that have windows to open.
![Apicture taken out of a railway carraiges window. The window is half open and you can see a beautiful landscape outside. It's moundtains and a river with a blue sky.](https://diningcar.at/wp-content/uploads/2022/01/fensterauf2021-1200x800.jpg)
It is a Hashtag widely used on Twitter by railway enthusiasts, which refers to trains with carriages that have windows to open.