The British Rail Class 25 diesel locomotives were
also known as Sulzer Type 2. In total, 327 locomotives
of this type were built between 1961 and 1967.
The Class 24 locomotives were the precursor of the Class 25 design but after the delivery of their first few units it became apparent that the speed ceiling of 75 mph (121 km/h) was unduly restrictive and the provision of a bit more power would be advantageous. In the course of normal development the power output of the Sulzer six-cylinder engine had been increased by 90 hp (67 kW) to give a continuous traction output of 1,250 bhp (930 kW) at 750 rpm by the introduction of charge air cooling and the first locomotives to use this became known as Class 25 locomotives.
The first 25 locos became known as Class 25/0 and were built at the BR Darlington works using the newer 1,250 hp (930 kW) "B" engine, modified generator assembly and traction motors. This increase in power was obtained from an air/water free flow intercooler fitted between a higher capacity pressure charger and inlet manifold, included within the normal cooling circuit to maintain simplicity. The cylinder head was also modified and strengthened.
The Class 25/1 locomotives were built at BR Darlington and Derby locomotive works. They featured the new AEI 253AY traction motor, a result of the collaboration between BTH, MV and American builder Alco. For Class 25 locomotives these lighter motors meant the discontinuance of other weight saving measures being built into the design. They were highly rated in an attempt to overcome the loss of tractive effort normally found on starting.
No. D5209 was built by BR at Derby in 1963 and went new to Toton Depot in Nottinghamshire. After 24 years service it was withdrawn from Crewe on 24the March 1987. After being sold for scrap to a dealer in Leicestershire, a private group of individuals purchased the engine for use on the Worth Valley Railway. This group in turn donated the engine to the Keighley & Worth Valley Railway Preservation Society and worked its first trail runs on the KWVR on 10th October 1987.
Many thanks to Wikepedia and KWVR for the above notes
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