Tom is undoubtable correct in saying that Bulleid pacifics didn't actually go through carriage washers, however, several writers did quote this as a reason given for their shaped casing...... DW Winkworth P20 (Bulleid's Pacifics) "The whole of the engine from the wheels up was swathed in a casing, independent of the boiler, described by the designer as "air-smoothing", to allow cleaning through carriage washing plants. Flat at the top, it even had a pivoted cover to slide over the chimney!" B Haresnape P19 (Bulleid Locomotives) "With a most unorthodox appearance, described as air-smoothed. (and intended to allow cleaning through carriage washing plants). RJ Mannion P55 (The Southern Pacifics) believes that although often repeated, the idea of passage through carriage cleaning plant was not the real reason. He believes that the casing was originally designed as "streamlined" as per LNER and LMS. This would have appealed to the publicity conscious Southern Railway board. However with the advent of WW2 such frivolity was replaced by the "Air-smoothed" easier to keep clean (- possibly in existing carriage washers...) mixed traffic engines. He was also dismissive of the chimney cover being used in sheds as it may well be forgotten and left in place.... but he thought that it was a feature only early on and it was discontinued after smoke box modifications ( cowled front?). C Boocock P 51,52 (Oliver Bulleid's Locomotive) believes air-smoothed was to convince publicity people that it was streamlined when it was not. However "What the all-over , air-smoothed casing on a Merchant Navy did do was to enable the designer to eliminate yet more metal structures such as side footplates and valences and the brackets to hold them.....the overall casing was shaped to fit the outline of the wide firebox..." It would seem that there were several reasons that could have been influencing the shape of the boiler casing.....as with many engineering situations.