I was remembering that, back in 1987/1988, we took this (once-a-week?) service from Ostende to Warsaw.
The train was made up of various national carriages…at various points along the route, the train re-formed to branch out, north and south, etc.
The Warsaw carriage was excruciatingly uncomfortable…they had dessigned it as such.
On the return we simply took a berth in the Russian carriages with bed and samovar!
Crossing into Eastern Germany was weird…they kept searching the train.