 Locomotive exchanges were commissioned by the fledgling British Railways ( BR ) during 1948 , to test the best and worst aspects of locomotive design within the Big Four railway companies that had existed before nationalisation . The research gained from operating the best designs of the GWR , LMS , LNER and Southern railways on different areas of the British Railways network paved the way for several new classes of standardised locomotives to be constructed . These new locomotive designs were intended to replace some of the ageing designs inherited by British Railways . 
