In 1946 Frederick Hawksworth, Collett’s successor, introduced a higher degree of superheat to the Castle boiler with resulting increased economy in water consumption.
 From 1956 the fitting of double chimneys to selected engines, combined with larger superheaters, further enhanced their capacity for sustained high-speed performance.
The fastest recorded speed of a Castle Class engine was 102 mph achieved by 7018 Drysllwyn Castle at Little Somerford in April 1958 while hauling The Bristolian in the Up direction from Bristol to London.
The non-stop run over 117.6 miles took 93 minutes 50 seconds, an average speed of more than 75 mph.
In 1946 Frederick Hawksworth, Collett’s successor, introduced a higher degree of superheat to the Castle boiler with resulting increased economy in water consumption.  From 1956 the fitting of double chimneys to selected engines, combined with larger superheaters, further enhanced their capacity for sustained high-speed performance. The fastest recorded speed of a Castle Class engine was 102 mph achieved by 7018 Drysllwyn Castle at Little Somerford in April 1958 while hauling The Bristolian in the Up direction from Bristol to London. The non-stop run over 117.6 miles took 93 minutes 50 seconds, an average speed of more than 75 mph.
GWR 4073 Class