On GCI coaches, a side corridor was provided, at the inconvenience of reducing seating capacity; from 96 to 80 in 3rd class.
Also the Chemin de Fer à vapeur des Trois Vallées operates a few of these cars, as do some heritage railways in The Netherlands and Luxemburg.
These coaches were the mainstay of Belgian passenger rail traffic just after WW2; they can be seen in several comic books of the era.
One example is the Piet Pienter en Bert Bibber album Het raadsel van de Schimmenburcht; another is the Suske en Wiske story De Bokkerijders.
Paul Delvaux showed GCI carriages in his Station Forestière painting of 1960.