The A4 Pacifics were designed for high-speed passenger services. The application of internal streamlining to the steam circuit, higher boiler pressure and the extension of the firebox to form a combustion chamber all contributed to a more efficient locomotive than the A3; consumption of coal and water were reduced. A further design improvement was fitting a Kylchap double-chimney, first on 4468 "Mallard" in March 1938. The double-chimney improved the capability of the locomotives further, and the last three locomotives of the class (4901 "Capercaillie", 4902 "Seagull" and 4903 "Peregrine") were fitted with the Kylchap exhaust from new and the rest of the class acquired it in the late 1950s.