The versatile bi-mode locomotive that solved the Southern Region's electrification dilemma
In short: How do you run heavy freight through non-electrified yards without abandoning the efficiency of the third rail? The British Rail Class 73 was engineered to solve this exact puzzle, using a unique dual-power system to bridge the gap between electric main lines and diesel-only sidings.
The development of the Class 73 was driven by the limitations of the Southern Region's 650/750 V DC third rail system. While highly efficient for passenger electric multiple units, the third rail posed significant hazards in goods yards and required gaps for level crossings. Early attempts to solve these issues involved experimental locomotives with large flywheels to maintain momentum over gaps, or the installation of overhead wires in specific yards. However, these solutions were costly and limited operational flexibility.
In the early 1960s, engineers developed the 'electro-diesel' concept to create a truly versatile machine. The Class 73 was designed to utilize the 750 V DC third rail for high-power main line work, while an onboard 600 hp English Electric diesel engine provided enough power to move freight at slow speeds within non-electrified yards. To ensure safety, the locomotives featured retractable third rail pick-up shoes, preventing the accidental energization of dead rail sections during track maintenance or when moving under diesel power.
The class was produced in two distinct batches. The first six, built at Eastleigh Carriage and Wagon Works during 1961–1962, were classified as type JA and built to the Hastings Line gauge. A larger production run of 43 locomotives followed from English Electric's Vulcan Foundry between 1965 and 1967, classified as type JB. These later models offered higher maximum speeds of 90 mph compared to the 80 mph limit of the prototypes. Despite the technical differences between the JA and JB batches, the Class 73 proved remarkably durable, with many units remaining in service for over 64 years.
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- Summary by Intellect · source: British Rail Class 73
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