British Rail Class 73

The BR Class 73 electro-diesel was a family of forty-nine Bo-Bo locomotives built for the Southern Region between 1962 and 1967, capable of drawing power from the 750 V DC third-rail electrification that covered most of the Southern network or, where third-rail supply was unavailable, operating as a conventional diesel-electric using an English Electric 4SRKT engine producing 600 hp. This dual-mode capability — making the Class 73 the first production electro-diesel in Britain — gave it unique operational flexibility on a region where electrified and non-electrified routes interleaved in complex patterns, allowing a single locomotive type to work through from electrified main lines onto non-electrified sidings, branch goods workings, and departmental duties without changing traction.

Two prototype electro-diesels, classified Class 70, had been built in 1962 to test the concept; the production Class 73 followed in 1965–67, built at Vulcan Foundry (Newton-le-Willows) for the electrical equipment and with mechanical parts by English Electric. The 49 production locomotives were numbered E6001–E6049, later renumbered 73001–73049 under TOPS. They entered service on Southern Region duties including empty coaching stock movements, Gatwick Express services, parcels and mail, Boat Train workings, and infrastructure duties — wherever the ability to work off third-rail and then continue as a diesel was operationally valuable.

The Class 73's most high-profile regular duty was the Gatwick Express from London Victoria to Gatwick Airport, which the class worked from 1984 to 2005, at times in push-pull formation with a Driving Van Trailer. The 730 V DC capability allowed fast, smooth running on the electrified Brighton main line, while the diesel mode was available for stock movements and empty workings off the third rail.

Under privatisation the Class 73 fleet was distributed among several operators. Network Rail uses examples for infrastructure and test train duties, exploiting the dual-mode capability on routes where third rail ends and diesel operation begins. GB Railfreight and other operators have retained Class 73s for specialist duties. Several have been rebuilt as Class 73/9 with more powerful Cummins QSK19 diesel engines, transforming their diesel-mode capability from the modest 600 hp of the original to around 1,600 hp and making them genuinely dual-mode in the modern sense. As of 2026 the Class 73 remains in active service in various configurations, sixty years after the first production examples entered traffic.