The Charing Cross, Euston and Hampstead Railway was opened on 22 June 1907; and known as the Hampstead tube. It linked the northern suburbs to central London through deep tunnels under Hampstead Heath. The difficulty of running steam trains in narrow tunnels meant that the trains were originally intended to be hauled by cables; electrical power was used instead with a third rail system. This was the first major railway to adopt the 500 volt, third rail system. The system adopted unique engineering solutions to the problems of the City of London, boring tunnels one above the other, and adopting a steep gradient to dive beneath the River Thames.
By 1926, an extension opened to Kennington meeting the City and South London Railway. This led to a merger between the two companies and the introduction of Standard Stock trains. The companies were taken into public ownership on 1 July 1933, by the London Transport Board; forming the Northern Line of the modern tube service.
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