In the fifties, British Railways searched for a way to replace the fastest steam-hauled express services on the East Coast Main Line between King's Cross and Edinburgh. Since electrification of the complete ECML was not foreseeable in the near future, this had to be a diesel locomotive that could reach a higher average speed than the streamlined A4 Pacific. The answer was the DP1 prototype completed by English Electric in 1955. It was the most powerful single-unit diesel locomotive in the world at the time of its introduction into service.

The solution for getting as much power as possible from a six-axle locomotive that was not too heavy for high-speed running was the Napier Deltic engine. This was a two-stroke opposed-piston diesel engine inspired by the German Junkers engines of the thirties. In this case, it was combined of three banks of six cylinders each which were mounted in a triangular layout with a total of three crankshafts. Although not turbocharged in its original form, it could deliver 1,650 hp with its high running speed of 1,500 PRM at a low weight compared to other diesels of the time. Each locomotive had two of these engines with one generator each.

In 1961 and 1962, 22 production locomotives were completed. Officially called “English Electric Type 5”, they were usually called “Deltics” and later became BR class 55. Apart from their power, they became famous for their appearance with their great length and their bulldog noses inspired from American diesel locomotives. Although officially limited to a top speed of 100 mph or 161 km/h, they could complete the journey of the “Flying Scotsman” in six and a half hours, half an hour faster than the A4, due to their faster acceleration.

In the years after the introduction of the Deltic, the ECML was upgraded for higher running speeds. In daily service, they sometimes even exceeded 100 mph, further decreasing the running times. The highest speeds recorded were 117 mph (188 km/h) on the level and 125 mph (201 km/h) on downward stretches.

A downside of the Deltic engine was its complexity and complicated maintenance, so that more engines had to be held in spare to ensure the availability of the locomotives. With the introduction of the class 43 High Speed Train on the ECML, the Deltics became superfluous. After only a short time in secondary services, they were all withdrawn in 1980 and 1981 after less than 20 years in service. Six have been preserved, all of which have run in preservation.