British Rails' Class 23 was a small class of 10 locomotives numberd D5900 - D5909 powered by a single 9-cylinder Napier Deltic engine set at 1100 bhp - a smaller version of the power units fitted to the well-known Deltic class 55 locomotives, that carried two 18-cylinder engines. Having a similar body styling as well led to the smaller locos quickly being dubbed 'Baby Deltics'.
Sent new to the London end of the Eastern region in the spring of 1959 the locomotives worked suburban services from Kings Cross, included some express trains. Naturally there were teething troubles, but the locomotives were generally capable. However, the complex Deltic engines had been designed originally for motor gun boats, where prolonged running at constant high speeds was demanded, and they proved troublesome under the start-stop demands of suburban life. By the autumn of 1960 the rate of engine failures caused serious concern and while this was being addressed class was slowly collected at Stratford shed, pending a decision as to its future. A refurbishment and modification plan was implemented, including cosmetic alterations to the nose end, removal of the the gangway connection and the creation of a 4-character train describer box. The class returned to service, proving generally reliable.
In the late 1960s BR reviewed the performance of it's 'new' diesel locomotives, as a 'second generation' was coming into production. Under the National Traction Plan each class was examined, with an emphasis on standardisation and economy in operation. Small classes of locomotives were effectively doomed and the 'Baby Deltics' were withdrawn between 1968 and 1971, barely outlasting the last steam locomotives and with only one class member receiving the new standard blue livery. This proved not to be the final chapter however, as the Railway Technical Centre was looking for locomotives to power research trains. D5901 was chosen and worked in the departmental fleet until 1975, being cut up in 1977.