The British Rail Class 25, also known as the Sulzer Type 2, is a class of 327 diesel locomotives built between 1961 and 1967 for British Rail. They were numbered in two series, D5151–D5299 and D7500–D7677. Background 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 was unduly restrictive and the provision of additional power would be advantageous. In the course of normal development the power output of the Sulzer six-cylinder engine had been increased by to give a continuous traction output of at 750 rpm by the introduction of charge air cooling and the first locomotives to use this became known as Class 25 locomotives. The Class 25s were primarily designed for freight work, but a significant number were fitted with boilers for heating passenger trains. Throughout the 1970s they could be found at work across the whole of the British Rail network although the Eastern and Southern Regions never had a long-term allocation. Though regular performers into the early 1980s on Crewe–Cardiff passenger trains, they are best known in that respect for their use on the summer Saturday trains to Aberystwyth, a task they relinquished in 1984. The final Class 25 locomotive was withdrawn from operational service in March 1987 although it continued to be used on enthusiast specials until March 1991. Classification Class 25/0 Number sequence (original) D5151–D5175, (TOPS) 25 001–25 025 The first 25 locos became known as Class 25/0 and were built at the BR Darlington works using the newer "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 BTH generator, type RTB 15656, was rated as , 750/545 V, 1090/1500 A at 750 rpm, only slightly different from that used in the earlier Class 24s (Note that all Class 25 locomotives used a generator designated as BTH RTB 15656, but its rating and characteristics changed over time). The generator supplied four BTH 137BX traction motors connected in parallel and rated , 545 V, 375 A at 560 rpm with a gear ratio of 18:79 (to give a maximum speed). Maximum tractive effort was and continuous