Maunsell began experimenting with the use of diesel locomotives for yard shunting in 1937. He ordered three locomotives, which proved to be successful, but his retirement and the onset of the Second World War prevented further development. Bulleid adapted and improved the design but his class did not appear until 1949, after nationalisation. Bulleid also designed a class of main line diesel-electric locomotives, continuing to push back the boundaries of contemporary locomotive design and established practice, but this was built by British Railways.