Editing GWR 4073 Class (section)

==Subsequent modification== 
[[File:5043 Earl of Mount Edgcumbe parked up around the turntable.jpg|thumb|[[GWR 4073 Class 5043 Earl of Mount Edgcumbe|5043 ''Earl of Mount Edgcumbe'']] is one of two preserved Castles to be fitted with a [[double chimney]]]]
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 chimney]]s 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&nbsp;mph achieved by 7018 ''Drysllwyn Castle'' at [[Little Somerford]] in April 1958 while hauling ''[[The Bristolian (train)|The Bristolian]]'' from [[Bristol Temple Meads railway station|Bristol]] to London. The non-stop run over 117.6 miles took 93 minutes 50 seconds, an average speed of more than 75&nbsp;mph.<ref>{{cite book|title=Great Western|last1=Allen|first1=Cecil|date=1962|publisher=Ian Allan|pages=64}}</ref>

=== Oil firing ===
{{Main|GWR oil burning steam locomotives}}
Between 1946 and 1948 five engines—100A1, 5039, 5079, 5083 and 5091—were converted to oil-firing, but were soon restored to burn coal. Eleven [[GWR 4900 Class|Halls]] were also temporarily converted.{{sfnb |Haresnape |1978 |pp=27, 33}}