These coaches were built to a standard 5½ compartment design for use on premier expresses. All four diagrams were externally identical. The D1920 prototype had three per side seating whilst the first production batch to D1909 had luxury two per side seating. D1930 had three per side seating and a different arrangement of the non-smoking compartments. D1960 had two per side seating and pressure ventilation for use in the Coronation Scot.