The Advent of Electric Traction
The possibility of electric traction was first demonstrated by a Scotsman called Davidson in 1834 but it was not until the Berlin Exhibition of 1879 that the idea was developed far enough to show that it could be a practical challenger to steam.
The obvious advantages of electric traction over steam for underground railways attracted the attention of many engineers and operators around the world in the last decade of the nineteenth century.
The first ?Tube' line to be built in London
London Railway between King William Street and Stockwell in 1890 using electric traction. This was followed within ten years by the construction of the Central London Railway from Shepherds Bush to Bank, also using electric traction. Other tube lines followed rapidly, all of which were in corporate into today's London Underground Most of these early tube lines followed the main line practice of a single locomotive pulling non-powered carriages or cars. The City & South London locomotives were small four wheeled vehicles whereas the Central London Locomotives were a much larger ?camel driving axles mounted in two bogeys.
During the first decade of the twentieth century all of the London tube lines departed from the principle of single locomotive hauling to using a number of motorcars along the length of the train. This has considerable advantage for rapid transit trains, not the least of which is to distribute both traction and braking along the full length of the train. This has the effect of improving both acceleration and braking, which is important on lines where there are frequent stops.
For the same reasons many main line railways have now come away from the use of locomotives for suburban and stopping services and have adopted multiple units with motors distributed along the length of the train.
Development of Electric Traction
The suburban and underground railways that were built or electrified in the early part of the twentieth century adopted a medium voltage direct current supply system which involved fairly costly fixed equipment but kept 34 Practical Railway Engineering the locomotives relatively simple and cheap. A large number of transformer ?sub-stations' were involved with comparatively at track level. Technology was very similar to the early electric tramways which were also powered with direct current.
In the UK, London Underground and a large part of the Southern Region of British Railways adopted DC electric traction many years before the rest of BR converted from steam power to diesel power or seriously considered large scale electrification.
Overhead supply of high voltage alternating current was pioneered largely in Switzerland after the First World the normal system of electrification on the Continent.
High voltage AC electrification was not introduced to British Railways until after the Second World War since when it has become the preferred system for surface railways. High speed AC electric locomotives have a high power/weight ratio as they carry no heavy fuel.