 The copper firebox round-top boilers the original eleven locomotives were built with, prone to priming if too much water was carried, were replaced with all-steel boilers featuring Belpaire pattern fireboxes.The new boiler design also featured a combustion chamber and thermic syphons to increase power and efficiency.The VR was so satisfied with the performance of the revised X class all-steel boiler design, a shortened barrel version was considered during the design phase of the R class 4-6-4 express passenger locomotives of 1951.