British Rail Class 45 (Sulzer Type 4) diesel locomotives

Design and characteristics

The Class 45 was built around a Sulzer diesel engine layout and diesel-electric transmission, earning the informal name Sulzer Type 4. The design emphasized reliability and maintainability for sustained mainline use. When new, most examples were fitted with steam heating boilers to provide carriage heating for passenger services, and they could be coupled into multiple because they supported multi-unit working, allowing two or more locomotives to be controlled from a single cab.

Service history and deployment

Introduced in the early 1960s, Class 45s quickly undertook express and secondary passenger services as well as freight duties. They operated across the Midland and other main line corridors, forming a workhorse element of diesel traction while British Rail modernized its fleet. In the early 1970s a programme converted about fifty locomotives to fit electric train supply in place of their original steam boilers; this adaptation allowed them to work air-conditioned coaching stock and electrified train sets on routes such as the Midland Main Line out of London St Pancras to destinations including Nottingham, Derby and Sheffield.

Operations and roles

• Passenger expresses on regional and intercity services, particularly where electric multiple units were not available.
• Secondary passenger duties and charter trains, benefiting from their heating and multiple-working capability.
• Mixed and freight trains where road haulage or other traction was less suitable; their mainline gearing made them useful on longer-distance workings.

Later years and legacy

By the 1980s newer classes and electrification programmes reduced the operational need for Class 45 locomotives, and withdrawals accelerated. The last members of the class had been retired from regular service by 1989. Several examples remain preserved on heritage railways and are maintained by enthusiast groups, where they appear on special trains and gala events, preserving the engineering and operational character of British diesel traction from the mid-20th century.

Notable distinctions

Compared with their immediate predecessors, the Class 45s combined Sulzer engine technology with adaptations that allowed both steam and later electric train heating, reflecting the transition period of rolling stock they served. They are remembered for solid performance across a range of duties and for playing a bridging role between early diesel classes and later more powerful or specialized locomotives.

For further technical details, preservation status and operation histories consult enthusiast and archival resources via the links above or specialist publications on British diesel traction.