33026 'Seafire' and 33030 at Crediton in January 1993. ©Gordon Edgar
The British Rail Class 33, also known as the BRCW Type 3 or "Crompton", is a class of Bo-Bo diesel-electric locomotives. They were produced as a more powerful Type 3 development of the Class 26. This was achieved, quite simply, by removing the steam heating boiler and fitting a larger 8 cylinder version of the previous 6 cylinder engine. This was possible because of the traffic requirements of the Southern Region; locomotive-hauled passenger traffic depended on seasonal tourist traffic and was heavier in the summer, when carriage heating was not needed. In the winter, their expected use was to be for freight. Thus, they became the most powerful BR Bo-Bo diesel locomotive. The perennially unreliable steam heating boiler could also be avoided. A total of 98 were built by the Birmingham Railway Carriage and Wagon Company (BRCW) and they were known as "Cromptons" after the Crompton Parkinson electrical equipment installed in them.
(Information provided via Wikipedia)