be called to put the fire out, with cold water coming into contact with the hot boiler, causing stress to the casing. Many photographs show an unmodified locomotive with a 'buckled' (warped) casing, the result of a lagging fire. High fuel consumption. This became very apparent in the 1948 Locomotive Exchange Trials and at trials at the Rugby locomotive testing plant in 1952. This was largely attributed to the variability of valve events due to the chain-driven valve gear. Restricted driver visibility due to the air-smoothed casing. The exhaust problem was never adequately resolved, and continued to beat down onto the air-smoothed casing when the engine was on the move, obscuring the driver's vision from the cab. As a result of these problems, in 1954 serious consideration was given to scrapping the class and replacing them with Britannia class locomotives. However, the locomotives had excellent boilers and several other good features and so the decision was taken to rebuild them, removing several of Bulleid's less successful ideas. Modification Partially because of the Crewkerne incident, and due to the incessant modification of Bulleid's original design, British Railways took the decision to rebuild the entire class to a more conventional design by R. G. Jarvis, adopting many features from the BR 'Standard' locomotive classes that had been introduced since 1950. The air-smoothed casing was removed and replaced with conventional boiler cladding, and the chain-driven valve gear was replaced with three separate sets of Walschaerts valve gear. The rebuilds were provided with a completely revised cylindrical smokebox, a new Lord Nelson-type chimney and LMS-style smoke deflectors. Together with the lack of air-smoothed casing, these helped reduce the problem of smoke and steam obscuring the driver's vision of the line. The fast-moving and unpredictable Bulleid steam reverser was replaced with a screw-link version, whilst the mechanical lubricators were moved to the footplates along the boiler sides. Sanding was also added to the leading driving axle, whilst rearward application was incorporated to the middle driving axle. The first 'modified' locomotive to be released from Eastleigh was 35018 British India Line in 1956. The final example, 35028 Clan Line, was completed in 1960. The success of the modification programme for the Merchant Navy class was also to influence the design of the future modification of 60 'Light Pacifics'. Performance of the modified locomotives There is no doubt that rebuilding the class solved most of the maintenance