We’ve had electric trains in the UK since the 1890s, and diesel locomotives first started to appear in the 1930s. But for the last fifty years, the UK has had one unusual locomotive: the boxy Class 73 Electro-Diesel. These rather uncharismatic locos have a unique talent – they can run on electrical power from the third rail, or on their own power from a diesel generator. They were built for the Southern Region of British Rail mostly to handle freight, but they did also get used to haul the Boat Trains that ran from London to the Channel ports. The 73s are still in service, rescuing dead EMUs and handling freight and engineering trains all over Southern England.
The electro-diesel concept didn’t really catch on because it was difficult to squeeze both power systems into the locomotives. The Class 73s are very successful, but they are primarily an electric locomotive: the diesel generator can only supply half the output power that’s available in electric mode. Improvements in both the efficiency of diesel engines and the compactness of power electronics means that these limitations can now be overcome, and we’re about to see a whole range of new electro-diesel trains enter service in the UK.
Freight operations on UK railways are mostly diesel-hauled at present, since many freight terminals are located on private sidings that aren’t electrified. Most freight trains run point-to-point rather than being shunted in marshalling yards, so there’s often no sensible reason to switch from an electric loco to a diesel one. However, the rising cost of fuel and the fact that much more of the mainline network is to be electrified means that one freight operator, Direct Rail Services, has decided to order new ED locomotives. These are being made by Vossloh in Spain, and will be primarily electric locos, using 25kV overhead supply. In electric mode, the power output is 4MW (nearly 5,500 horsepower). However, they have a “last mile” diesel capability using a 700kW (930hp) onboard generator – just enough to shunt gently down a siding or short branch line. For example, the BMW-Mini factory here in Oxford is served by its own short branch (which is all that’s left of a line that once went to High Wycombe), and long car-carrier trains are shunted a couple of miles to the main line just south of Oxford station. Once the “Electric Spine” programme is completed, trains from the factory could change to electric power and run at full speed as far as Southampton docks, or join the main electric network in the Midlands.
Passenger services will also be moving to electro-diesel in the next few years. The Intercity Express Programme is a project to replace the UK’s fleet of IC125 High Speed Trains, which date from 1975. Hitachi Rail have won the tender, and will be delivering two fleets of fast, long-distance passenger trains. The Class 800 trains are described as “bi-mode” and are electro-diesel sets which are designed to operate at full speed in diesel mode. These will be used for services to destinations “beyond the wires” – such as London-Aberdeen. The sister Class 801 trains are designed for service operation only on electrified routes, but will still have a small diesel generator set that can be used to shunt in depots and deal with power failures. Modern trains are air-conditioned and a power failure can result in severe discomfort for passengers, so the new trains will be able to start their generators to keep the lights and air conditioning working, and crawl slowly to the next station.
Finally, the venerable 73s are not being forgotten – two units are currently in the workshops of Railway Vehicle Engineering Ltd in Derby being fitted with modern Cummins diesel generator sets. These are much more powerful than the units they replace, meaning that these two new 73s will have equal performance in diesel and electric modes. I suspect that these will be really useful engines…