The class 23, originally known as D5900, was a class of ten type 2 diesel locomotives built by English Electric in 1959. It was powered by a Napier Deltic engine with nine cylinders that was basically one half of one of the 18-cylinder engines of the class 55. While the latter had an output of 1,650 hp per engine, the smaller one managed to produce 1,100 hp thanks to turbocharging. The light Deltic engine was chosen to reduce the weight of the locomotive. But the envisaged axle load of 16 tons was exceeded by the weight of several accessories. This had been found out after the first locomotive was completed and some parts were made lighter, but in the end each locomotive still weighed more than 72 tons.

They were placed into outer suburban service starting from King's Cross and earned the nickname “Baby Deltics”. In comparison with heavier, simpler engines, the complex and fast-running Deltics had a much higher failure rate and many engines had to be replaced. Although the engines were refurbished in the early sixties to become more reliable, this small class was judged non-standard when British Railways tried to reduce the number of different diesel locomotive types. So they were withdrawn between 1968 and 1971 and only D5901 could still work on until 1977 in departmental service. All locomotives were scrapped, but the Baby Deltic Project is currently building a new class 23 locomotive from a surviving engine and the remains of a class 37 locomotive that has been shortened and provided with class 20 bogies.