expediency. This resulted in several class members having a horizontal strengthening rib running down the length of the casing. The final Southern Railway-initiated experiment involved equipping 21C5 Canadian Pacific with a Berkeley mechanical stoker imported from Canada. Little improvement in performance was seen when trialled under British Railways auspices in 1948 and the locomotive was re-converted to hand-firing. As mentioned, the British Railways batch had detail differences to previous versions. The most significant modification was the reduction of weight using lighter materials unavailable during wartime. From 1952 the air-smoothed casing ahead of the cylinders was removed to ease maintenance and lubrication. This coincided with the removal of the tender 'raves' on all locomotives, as they quickly rotted, obstructed the packing of coal into the bunker and restricted the driver's view when reversing the locomotive. The resultant 'cut-down' tender included new, enclosed storage for fire-irons, revised step ladders and glass spectacle plates to protect the crew from flying coal dust when running tender-first. Performance of the unmodified locomotives The new locomotives demonstrated that they could generate enormous power using mediocre quality fuel, due largely to Bulleid's excellent boiler. They also ran very smoothly at high speed. Partly as a result of having so many novel features, the first few years of service by the Merchant Navy class were beset by a variety of technical problems. Some of these were merely teething troubles, but others remained with the class throughout their working lives. These may be summarised as follows: Adhesion problems. The locomotives were often prone to wheelslip, and required very careful driving when starting a heavy train from rest, but once into their stride they were noted for their free running, excellent steam production and being remarkably stable when hauling heavy expresses. Maintenance problems. The chain driven valve gear proved to be expensive to maintain and subject to rapid wear. Leaks from the oil bath onto the wheels caused oil to splash onto the boiler lagging in service. Once saturated with oil, the lagging attracted coal dust and ash which provided a combustible material, and as a result of the heavy braking of the locomotives, sparks would set the lagging on fire underneath the air-smoothed casing. The fires were also attributed to oil overflowing from axlebox lubricators onto the wheels when stationary to be flung upwards into the boiler lagging in service. In either case, the local fire brigade would invariably