Joseph Armstrong's early death in 1877 meant that the next phase of motive power design was the responsibility of William Dean, his assistant and successor. Dean went on to develop express 4-4-0 types, but the familiar 4-6-0s of later years were initially introduced by the next engineer, George Jackson Churchward. He was also responsible for the introduction of self-propelled Steam Rail Motors for suburban and light branch line passenger trains. Next came Charles Collett in 1921; he standardised the many types of locomotives then in service, producing the iconic Castle and Kings. He also introduced diesel power in the form of streamlined rail cars in 1934. The final engineer was Frederick Hawksworth who took control in 1941 and produced GWR-design locomotives until after nationalisation in 1948.