 The BR Standard Class 7 , otherwise known as the Britannia Class , is a class of 4 @-@ 6 @-@ 2 Pacific steam locomotive designed by Robert Riddles for use by British Railways for mixed traffic duties . Fifty @-@ five were constructed between 1951 and 1954 . The design was a result of the 1948 locomotive exchanges undertaken in advance of further locomotive classes being constructed . Three batches were constructed at Crewe Works , before the publication of the 1955 Modernisation Plan . 
