The Great Western Railway 2900 Class or Saint Class, which was built by the Great Western Railway's Swindon Works, incorporated several series of 2-cylinder passenger steam locomotives designed by George Jackson Churchward and built between 1902 and 1913 with differences in the dimensions. The majority of these were built as 4-6-0 locomotives; but thirteen examples were built as 4-4-2 (but converted to 4-6-0 during 1912/13). They proved to be a successful class which established the design principles for GWR 2-cylinder classes over the next fifty years. Background After finally converting the last broad gauge lines in 1892, the Great Western Railway (GWR) began a period of modernization as new cut-off lines shortened its routes to west of England, South Wales and Birmingham. During the first decade of the twentieth century the Chief Mechanical Engineer, George Jackson Churchward, designed or acquired a number of experimental locomotives with different wheel arrangements and boiler designs to help him plan for the future motive power needs of the railway. The first of these was a two-cylinder 4-6-0 locomotive, designed in 1901 whilst Churchward was still the Chief Assistant of his predecessor William Dean. Prototypes Between 1902 and 1905 Churchward built and tested three prototype locomotives with detail differences, before using the third as the basis for the production series. No. 100 The first prototype was completed at the Swindon Works of the GWR (Lot 132) in February 1902. It was numbered 100 and in June 1902 was named Dean (later William Dean) to mark the latter's retirement. The new design incorporated all of Churchward's current ideas including a domeless parallel boiler, raised Belpaire firebox, diameter outside cylinders with piston stroke, and boiler pressure of . The piston valves were driven by rocking levers actuated by the expansion link of Stephenson valve gear – this particular design was only used on no. 100. The parallel boiler was later replaced with a taper boiler, and then the first superheated taper boiler in 1910. Churchward had studied American boiler design, but he was also later influenced by continental practice in efficient motion design. From the Société Alsacienne de Constructions Mécaniques (SACM) a de Glehn 4-4-2 compound engine was ordered for comparative trials on the GWR. According to E.C. Poultney, No. 100 was the first 4-6-0 locomotive to have high enough boiler capacity and steam ports large enough to handle the steam flow required by large cylinders: "The