 Its total route length was 3800 miles (6116 km), much of which had been built to handle the coal traffic from south Wales.Though this appeared to be a great coup for the GWR, the coal traffic declined significantly as the use of coal as a naval fuel declined, and within a decade the GWR was itself the largest single user of Welsh coal.The 1920s also saw the introduction of the GWR's most famous locomotives – the Castle and King classes developed by C. B. Collett.