The British Rail Class 73 is a British electro-diesel locomotive. The type is unusual in that it can operate from the Southern Region's 650/750 V DC third-rail or an on-board diesel engine to allow it to operate on non-electrified routes. This makes it very versatile, although the diesel engine produces less power than is available from the third-rail supply so the locomotives are rarely operated outside of the former Southern Region of British Rail. It is one of the first bi-mode locomotives ever built. Following the withdrawal and scrapping of the more powerful Class 74 bi-mode locomotives in 1977, the Class 73 was unique on the British railway network until the introduction of the Class 88 bi-mode locomotives in 2017. Ten locomotives have been scrapped.

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The Southern Railway's expanding third rail electric passenger network (which had begun as far back as 1915) was until 1941 a purely passenger electric multiple unit (EMU) system. This was because it was necessary to have gaps in the third rail for level crossings, etc., which effectively prevented the use of electric locomotives on either passenger or freight. It was not until the arrival of Oliver Bulleid as Chief Mechanical Engineer of the Southern Railway, in 1937, that the problem was finally addressed when he teamed up with Alfred Raworth, the Chief Electrical Engineer, to solve the issue. To overcome the problem of gaps in the third rail three experimental locomotives were built (which later became the British Rail Class 70) which were fitted with large flywheels that maintained momentum long enough to avoid stalling in gaps. The second problem for freight train operation by electric locomotives was the serious hazard that would result if the 750 V DC third rail was laid in goods yards, as this would not only be a danger to personnel on the ground but also present some complex issues loading and unloading many types of freight wagons. The initial solution was to install simple tramway-type overhead wires to carry the 750 V supply in certain yards and add a pantograph on the locomotive roofs.

As a continuation of the Southern Railway's policy of electrification, British Railways then began electrifying the main lines to the Kent Coast as part of the 1955 Modernisation Plan. In addition to the few hundred new EMUs required, a small fleet of 25 Bo-Bo electric locomotives of 2,552 hp (1,903 kW) classed type "HA" (later Class 71) were built to deal with freight, parcels, and the few remaining locomotive-hauled passenger trains in Kent, such as the "Night Ferry" and "Golden Arrow" services. These locomotives also had a flywheel and pantograph, and were able to work in the more important freight yards across Kent that were fitted with the simple 750 V overhead wire system. This system was brought into use across Kent between 1959 and 1961.

Although successful this system did require considerable extra cost, and maintenance, and still limited freight operations with the new locomotives to those goods yards fitted with the catenary. Something more versatile was needed. Development and advances in both electric locomotive and diesel engine design in the early 1960s resulted in the Southern Region engineers beginning to consider the possibility of a combined electric and diesel locomotive. The requirement was for an electric locomotive with a similar power when using the electrified third rail to the already successful Type 3 Birmingham Railway Carriage and Wagon (BRCW) built diesel locomotives (later Class 33) then entering service on the Southern. This would be supported by adding a small diesel, engine powerful enough to move reasonable freight loads at slow speed within goods yards. The new locomotive design would also need retractable third rail pick up shoes. This was for two reasons: the first was safety in connection with track relaying jobs, avoiding the problem of bridging a gap and energising a dead section of third rail, and electrocuting track workers who might be in contact with the dead third rail section. The second was to allow locomotives under diesel power to move off and back onto third rail areas without risk of damaging "fixed" third rail pick up shoes.

The Southern Region engineers, having done all the initial design work, set about building a prototype batch of six new "electro-diesel" locomotives at Eastleigh Carriage and Wagon Works during 1961–2. The new locomotives had a respectable 1,600 hp on electric power, and an English Electric 600 hp diesel engine similar to those used in the Southern Region's diesel electric multiple Unit (DEMU) fleet. To improve versatility yet further, the locomotives were designed so that they could work in multiple with most of the Southern's electric and diesel multiple unit fleets as well as normal carriages and wagons. This meant the locomotives had to also be fitted with drop buckeye couplings and therefore the Pullman rubbing bar, and retractable buffers. The success of these prototype locomotives resulted in British Railways ordering a production batch of a further 43 locomotives from English Electric at the Vulcan Foundry in Newton-le-Willows. However minor technical differences prevented the prototypes (classified type "JA") working in multiple with the production examples (classified type "JB"). In all other respects, the new "electro-diesels" proved extremely versatile, to the point where many are still in service after a life of over 62 years.

• On 26 April 1968, locomotive E6023 was derailed at Earley, Berkshire when a set of points were moved by mistake.^[1]
• On 8 January 1972, locomotive E6027 collided with 4-BEP no 7004 at Horsham, injuring 15.^[2]
• On 12 October 1972, locomotive E6001 was hauling a freight train that ran into the rear of a passenger train at Wimbledon, London due to inattentiveness on the part of the driver. Twelve people were injured.^[3][4]
• On 16 January 1982, 73 115 was hauling a departmental train which overran signals and ran into the rear of a parcels train at East Croydon.^[5] The severely damaged locomotive was withdrawn and subsequently scrapped.^[6] Locomotive 73 006 was hauling the parcels train that was run into.^[5]

This class of 49 locomotives were built in two batches using English Electric components. The first six locomotives were built by BR at Eastleigh Carriage and Wagon Works and introduced to service during 1962; they were built to the Hastings Line gauge, numbered E6001-E6006 and classified as type JA.^[7] With the introduction of TOPS in 1968 they were to have been classified as Class 72, to differentiate from the later built units. However, instead they became Class 73/0. In the early 1970s the locomotives were renumbered 73001-73006.

Following successful trials of the initial locomotives, a production run of 43 locomotives were ordered as part of the Bournemouth Electrification and built by English Electric at their Vulcan Foundry between 1965 and 1967. They were initially classified as Class JB and numbered E6007-E6049.^[7] They differed slightly from the six earlier machines, most notably having an increased tractive effort as well as a higher maximum speed (90 mph as opposed to 80). Following the introduction of TOPS, they became Class 73/1 and were renumbered 73101-73142. One locomotive, E6027, had already been withdrawn following accident damage and so was not renumbered. Further changes were the use of large round Oleo buffers with a pneumatic withdrawal mechanism rather than the traditional coach style (oval) saddle buffer which relied on a pin and spring mechanism. As the JA examples came in for overhaul over the years, the saddle buffers were also replaced making visual identification of the differing machines almost impossible from a distance.

From new, all members of the class were fitted with the Pullman style rubbing plate between the buffers allowing them to close couple with Southern Region electro-pneumatically controlled electric multiple units and diesel electric multiple units for push-pull train operation – the reason for retractable buffers.

Being built to the Hastings Line gauge, the locomotives were able to be used on all routes of BR's Southern Region network.

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The design parameter of the Class 73 was to provide a secondary main line electric locomotive to the then Southern Regions "mixed traffic" requirements, with the added ability to deliver, shunt and collect freight to and from yards adjoining the electrified network under diesel power. The shunting requirement meant these locomotives had dual controls on both sides of the cab.

The limitations of a 1,600 hp electric locomotive meant that it was only capable of hauling maximum ten-coach trains to the proposed post-steam era Southern Region express timings, as were the similarly powered Type 3 BRCW Class 33 diesel locos with 1,550 hp. Southern express electric multiple units had, by comparison, at least 1,000 hp per four-car unit. To assume that the Class 73 was suitable to haul passenger or freight trains away from the electrified network with just a 600 hp diesel engine would be unrealistic. In fact the use of the slightly more powerful Class 74 electro-diesels, with 650 hp on diesel, to haul parcels trains up Poole Bank and work freight trains around the West London Line to Acton, Brent and Willesden, resulted in repeated failures; this was, therefore, a major contributory factor to that class's early demise.

There were eight third rail collector shoes on the Class 73 – four per bogie with two each side. These shoes were of the retractable type – a necessity for both safety and operational reasons, as described above. They could cope with all the then (1965) voltages of 660V and 750V as well as the 800V of the Bournemouth electrification beyond Pirbright Junction south of Woking (from autumn 1966). 660V was phased out in line with pre-BR type electric multiple unit (EMU) withdrawals, the last EMUs incapable of working in passenger service on anything greater than 660V being the 4SUB Bulleid design. Thus all BR post-1951 EMU designs could cope with 750V or 800V.

Gaps in the third rail at complex pointwork or level crossings mean that a short electric locomotive such as Class 73 is liable to momentarily lose power when passing over. This results in noticeable arcing if the driver does not shut off power. However, this rarely affects the locomotives, except to burn out the carbon brush pick-up shoes more quickly. The issue of arcing only became a problem when some of these locomotives were altered to work "Gatwick Express" services using modified Mark 2 coaching stock and 500 hp Gatwick Luggage Vans (GLVs), as 750V power jumpers between locos or units were banned on BR designs (the last EMUs to have such jumpers were the Bulleid designed 4SUBs). The Class 73s therefore could not benefit from the pick-up shoes at the opposite end of the train on the GLV. As a result of removing the last bank of resistance on the Class 73 locos to make these trains accelerate more quickly, arcing increased. Therefore, to reduce damage to other electrical equipment from the increased arcing, these particular locomotives had "flash guards" fitted on their bogies around the shoes. Stories of Class 73s catching fire were greatly exaggerated.

The Class 73 locomotives were very versatile, with dual cab controls; three brake systems (vacuum, air and EPB) as detailed below; a well-designed cab with good sighting that aided shunting; the ability to work fitted freight (fully braked) and unfitted freight (no train brakes) as well as locomotive-hauled coaching stock; the ability to work in multiple with blue star coded diesel locomotives; and the ability to operate in multiple with the Southern's diesel electric multiple unit fleet (Class 73/0) or its electric multiple unit fleet (Class 73/1).

Power supply

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Unlike the previous Southern Region Class 70 and Class 71 electric locomotives, these did not have a booster to maintain traction power over gaps in the conductor rail. However, they could move forward onto an electrified section under diesel power.

Diesel engine and generator

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The 600 horsepower (447 kW) English Electric 4SRKT Mk II diesel engine was less powerful, but more reliable than the 650 horsepower (485 kW) Paxman 6YJXL fitted to the later Class 74 electro-diesel locomotives.

Traction control

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The Class 73 was fitted with two separate power controllers on the driver's desk (and replicated on the secondman's side); one for electric or auxiliary power (under diesel power) and the other only for diesel power. The auxiliary power arrangement was to accommodate diesel operation when being driven from mainline EMU stock.

Several locomotives have been preserved on heritage railways, where perforce they run on their diesel engines: no preserved line has third rail electrification.

• P32AC-DM A United States locomotive with the same ability.