text: History 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  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.