The class 50 was a six-axle diesel-electric locomotive designed for hauling heavy express trains at speeds of up to 100 mph or 161 km/h. It was to be lighter than the class 40, but still more powerful. A prototype called DP2 was presented in 1962 that had the same body design as the Deltic, but was powered by a single English Electric 16CSVT delivering 2,700 hp. Series production was delayed until 1967 since the reliability of this engine still had to be proven.

The final production variant had a more boxy body and was built by English Electric at their Vulcan Foundry plant. They were originally numbered D400 to D449 and were named after ships of the Royal Navy. The distinct sound of their cooling fans earned them the nickname “Hoovers”. As planned, they were used on the West Coast Main Line between Crewe and Scotland. When electrification of this section was finished in 1974, they came to the Great Western Main Line and replaced the diesel-hydraulics of the Western Region.

Between 1979 and 1983 they were refurbished. In this process, the electrics were simplified, the rheostatic brakes were removed and the fans were modified. In the same period the class 43 High Speed Train replaced the class 50 on the GWML and they now moved to other lines. Their withdrawals took place in the late eighties and early nineties when the Sprinter DMUs were introduced. In their last years, 29 had received the Network South East livery. 18 were preserved, with twelve still being operational.