The compartment ceiling panels are of plastic material backed with ply or hardboard panels.1951 May, “British Railway Standard Coaches”, in Railway Magazine, page 328:
For convenience and versatility, no mass-produced wood paneling can beat hardboard—a tough, pliable, water-resistant material made from wood fibers.1980, Robert M. Jones, editor, Walls and Ceilings, Time-Life Books, →ISBN, page 67: