that the 16CSVT was unproven, and doubted its ability to produce the required power levels. Even after the delivery of DP2 in May 1962, any hope of future orders seemed over with the BTC's decision to standardise its Type 4 fleet on the Brush-Sulzer Class 47 design. The DP2 design earned a reprieve in 1965, when the National Traction Plan of that year identified a need for an additional 50 Type 4 locomotives. By this time the Class 47 fleet were starting to suffer serious problems with engine stress cracking, a problem which necessitated the derating of the fleet to 2,580 hp. At the same time, DP2 and its 16CSVT engine had proved far more reliable than expected. Negotiations took place with English Electric for a production batch of 50 locomotives for use on the Eastern Region. English Electric intended to build the new batch as similar to DP2 as possible but the British Railways Board (successor to the BTC) had produced a standard locomotive cab with a flat front and headcode box and also had specific requirements relating to the engine room and other equipment. English Electric produced several alternative front-end designs including one with a wrap-around windscreen but the standard front-end design was eventually adopted for the class. Production The complete production run of 50 locomotives was built in just over a year and numbered from D400 to D449. D400 entered service in October 1967 and deliveries were completed with D449 in November 1968. Unusually, the ownership of the locomotives remained with the manufacturer and they were operated by British Rail on a 10-year lease which included certain stipulations relating to availability. The D prefix was quickly dropped for all diesel engines after steam was banned from British Rail's mainline in 1968. The Class 50 engines were numbered as 400 to 449 from 1969 to 1974. Service The class was built for working passenger services on the West Coast Main Line (WCML) north of , to , , Carlisle, and Perth, occasionally reaching as far north as Aberdeen. Services south of Crewe would generally be worked by an electric locomotive, with the Class 50s taking over for the journeys that continued north. Initially trains were hauled by a single locomotive, but from May 1970 they were paired on 8 out of 34 locomotive diagrams covering Anglo-Scottish services north of Crewe, allowing greatly accelerated timings to be applied (including