Built in 1961 and 1962, the Class 55 Co-Co diesel locomotives were specifically designed for the high-speed express passenger services on the East Coast Main Line (ECML). The Class, built by English Electric, was powered by two 18-cylinder D18-25 series II type V Deltic engines, each rated at 1,650 hp continuous at 1500 rpm. This unique engine also powered the DP1 prototype that preceded the Class 55s, with the prototype being named ‘Deltic’ after its engines, hence the Class 55 also became known as ‘Deltics’.
After the introduction of the HSTs, the Class 55s were deposed from many of their express duties, instead being placed on secondary services. It soon became clear that maintaining the Deltics for these secondary duties would not be economically viable meaning that the class were run with minimal maintenance until they failed, at which point they would be sent to Doncaster Works to be scrapped. Six locomotives were preserved, including 55 002 ‘The King's Own Yorkshire Light Infantry’ which was selected by the National Railway Museum to join the National Collection.