The later class 15 was a diesel-electric locomotive designed by British Thomson-Houston for the requirements designated “Type 1” in the 1955 Modernisation Plan. This meant that it was designed for light freight service and had to have an output of 1,000 hp or less. Furthermore, it had to be equipped for multiple working. The BTH design had a fast running 16-cylinder Paxman engine with 800 hp that required a considerable deal of maintenance. As BTH did not have the capacities to build these locomotives, the orders for 44 were subcontracted to the Yorkshire Engine Company and the Clayton Equipment Company.

They were used in the Eastern Region and around London. Usually they only worked in freight service and only had to haul passenger trains when other locomotives had failed. Withdrawals already commenced in the sixties due to the complex maintenance and poor reliability. In comparison, the class 20 offered 200 hp more with half the number of cylinders. All were withdrawn by 1972, but four were rebuilt to non-powered pre-heating units for passenger trains. Only one of these was still used after 1982, but one remained until 1989. One of these, D8233, is still existing today.