|The London standard|
In Britain it was the Low Voltage direct current (LVDC) system which predominated. Following its successful application on the City & South London, the Great Northern & City, the Mersey Railway and the Liverpool Overhead, in 1911 it was adopted by the London & North Western for the Euston to Watford line, and in 1916 for the North London Line. In 1913, after good experience on the Waterloo & City, the London & South Western adopted it for its suburban lines out of Waterloo.
It was the shared line of these two between Richmond and Gunnersbury which encouraged harmony of operating practices and what became known as the ‘London Standard’. Within this standard practice there was some variation in the number of conductor rails. Whereas on the surface lines a single conductor rail, the third rail, with current returning through the running rail was considered acceptable, in the confined areas of the underground railway, where there was close proximity to other underground channels such as gas and water pipes and electricity wiring it was considered safer to use a separate conductor rail, the fourth rail.
An exception to this general practice was on the London, Brighton & South Coast Railway where, in 1903 on grounds of cost, the decision was taken to employ overhead wire AC transmission. On amalgamation in 1923 the newly created southern Railway decided to employ the LVDC system and so the only extensive AC operation in Britain was lost.
There was an earlier similar example of AC distribution in Britain on the Midland Railway between Lancaster and Heysham. This was brought about as the railway had a power station generating AC for its harbour equipment and in 1906 the decision was taken to electrify the 10 mile railway. This was where tests were carried out in the 1950s to determine the future AC system to be employed on the main lines to Scotland.
A more detailed factsheet on the London standard is available here.