British Rail Class 25

The BR Class 25 was the most numerous of all the first-generation British diesel locomotive classes: 327 Type 2 Bo-Bo diesel-electric locomotives built between 1961 and 1967 by British Railways at Derby and Darlington Works, and by outside contractors, powered by Sulzer 6LDA28 or similar six-cylinder engines producing 1,250 hp. The Class 25 was the definitive BR Type 2 mixed-traffic locomotive, a class large enough to work virtually every secondary mixed-traffic duty on the national network and versatile enough to appear on passenger trains, express freight, parcels, ballast, and engineers' workings across all six BR regions.

The Class 25 was the production development of the earlier Class 24, sharing the same Sulzer engine and basic mechanical layout but incorporating improvements to the cab design, electrical equipment, and bodyside ventilation that addressed problems identified in service with the Class 24. The resulting locomotive was reliable, free-steaming — in the diesel sense — and economical on fuel, making it a favourite with operating departments despite its modest 1,250 hp power output and 90 mph maximum speed.

Working from depots including Crewe, Springs Branch (Wigan), Holbeck (Leeds), and many others, Class 25s hauled everything from the Manchester–Glasgow summer Saturday extras to the limestone traffic from the Derbyshire quarries. The class's versatility was legendary: in emergencies a Class 25 could be found substituting for almost any other diesel type on almost any duty. The distinctive low exhaust note — the 'rats' as enthusiasts called them, from the Sulzer engine sound — became one of the defining sounds of British railways in the 1970s.

Withdrawals began in 1980 as the newer Class 47 and 56 took over more duties, and the last examples were withdrawn by 1987. Several are preserved, with 25322 Tamworth Castle and 25059 among the best-known preserved examples.