appeared in 1906. One was the Caledonian Railway’s Cardean Class which was, at the time, the most powerful locomotive in Britain. The other was Churchward's four-cylinder GWR Star Class, which was developed and enlarged by Charles Collett as the GWR 4073 Castle class in 1923 and later also as the GWR 6000 King class in 1927. Other significant early express designs included: The LNWR's Prince of Wales Class, with 246 locomotives built between 1911 and 1921. The LNWR's Claughton Class, with 130 locomotives built between 1913 and 1924. The Class S69 of the Great Eastern Railway (GER), with 81 locomotives produced between 1912 and 1928. Robert Urie of the London and South Western Railway (LSWR) introduced three successful classes, the H15 class mixed traffic locomotives, introduced in 1914 and built until 1924, the N15 King Arthur class, with 74 locomotives built between 1919 and 1926, and the S15 class, with 45 locomotives built between 1920 and 1936. In 1907, Wilson Worsdell of the NER built ten W class T tank locomotives. These were all rebuilt to NER Class W1 4-6-2T Pacific between 1914 and 1917. Post-grouping era During the post-grouping era from 1923 to 1948, the wheel arrangement was used extensively by all of the Big Four British railway companies, especially by the Great Western Railway (GWR) and the London, Midland and Scottish Railway (LMS), who continued to develop new designs. However, from the early 1930s, demands for more power and improved performance from express passenger locomotives led to the widespread introduction of 4-6-2 Pacific locomotives, where the trailing axle could support a larger firebox. Since the reduced traction of the driving wheels was not a big disadvantage with relatively light passenger trains, the was displaced from top-rank express services on most of the railways where they had been used, with the exception of the GWR who continued to build both mixed-traffic and express passenger s until nationalisation in 1948. The GWR's 4073 Castle Class eventually consisted of 171 express passenger locomotives, built between 1923 and 1950. The design was enlarged as the GWR's 6000 King Class, with thirty locomotives built between 1927 and 1930. Several new mixed traffic were also introduced: The Southern Railway improved the LSWR's King Arthur class and introduced the Lord Nelson class, which was briefly the most powerful class in Britain. Sixteen locomotives were built between 1926 and 1929. The LMS introduced the 7P Royal