British Rails' Class 23 was a small class of 10 locomotives numberd D5900 - D5909 powered by the 9-cylinder Napier Deltic engine set at 1,100bhp. This was a smaller version of the power units fitted to the well-known Deltic class 55 locomotives, which carried two 18-cylinder engines. The similar body styling and power units 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, duties which included some express trains. Naturally there were teething troubles, but the locomotives were proving capable. The complex Deltic engines were however designed originally for motor gun boats, where prolonged running at constant high speeds was demanded. Very different to the start-stop regime of suburban trains and by the autumn of 1960 the rate of engine failures was causing serious concern. While the engine issues were addressed the class was slowly collected at Stratford shed as they suffered major failures, pending a decision as to their future. A refurbishment plan was implemented, with modifications to the engines to correct the causes of failures implemented by EE, plus cosmetic alteration of the nose ends, removing the gangway connecion and creating a 4-character train describer box. The class returned to service, now proving generally reliable.
In the late 1960s BR was looking carefully at the performance of it's 'new' diesel locomotives, as the 'second generation' of diesels 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. The Baby Deltics were withdrawn between 1968 and 1971, barely outlasting the last steam locomotives and with only one class member receiving the new blue livery.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 choosen and worked in the departmental fleet until 1975, being cut up in 1977.
A Barrow Hill based group is currently adapting a class 37 locomotive to recreate a Baby Deltic, using on orignal engine. More information here at
The Baby Deltic Project.