In 1923, the railways in the UK were rationalised into four big companies - the grouping. The GWR remained almost unchanged and locomotive development unaffected. Dean at the beginning of the 20th century, then Churchward, Collett and Hawksworth were Chief Mechanical Engineers who developed the GWR loco with its unique style. On nationalisation in 1947 GWR loco's were the only ones to retain their pre-nationalisation numbers.

Great Western Railway locomotives were generally very successful and many remained in service almost untouched through the years from nationalisation in 1947 until the end of steam traction. A Castle 4-6-0 sending up a column of smoke as it slipped and struggled to pull a heavy South Wales express away from Swindon was quite a sight.