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.