However, after WWI, the move to oil-firing was quickly extended to other areas, including the railways, which was a key strategic factor in the economic hardship which struck the coalfield after the war. Coal workings were over-expanded in the late nineteenth century, and the Welsh coal owners had failed to invest in mechanisation. By the inter-war period the South Wales Coalfield had the lowest productivity, highest costs and smallest profits in Britain. Hardship continued through the 1926 general strike, the great depression of the 1930s, World War II and thereafter. The 1937 novel "The Citadel" and the 1939 novel "How Green Was My Valley" (later filmed, with a wildly inaccurate "colliery village") describe such hardship, as do the poems of Idris Davies the miner, teacher and poet of Rhymney.