Wooden carriages were originally hauled by steam locomotives leading to smoke-filled stations and carriages, unpopular with passengers. At the start of the 20th century the District and Metropolitan were seeing increased competition in central London from the new electric underground tube lines and trams, and conversion to electric traction was seen as the way forward. Experiments were carried out on the Earl's Court to High Street Kensington section, and a jointly-owned six-carriage train began passenger service in 1900. Following this an AC system was suggested, and this was accepted by both parties. However, the District was looking for a way to raise the finance needed and in 1901 found an investor, the American Charles Yerkes. He formed the Underground Electric Railways of London (UERL), and his experience in the United States led him to favour DC, with third-rail pick-up similar to that in use on the City & South London Railway and Central London Railway. After arbitration by the Board of Trade the DC system was taken up, and the railways began electrifying the routes, using multiple-unit stock.
The District and Metropolitan Railways bought different designs of electric multiple unit. Both had open saloons; the Metropolitan trains with gated ends, the District B Stock with sliding doors in the middle of each car. When their introduction was attempted on 1 July 1905, a Metropolitan train overturned the third rail on the District Railway, requiring all Metropolitan trains to be modified before running again on the District lines. Full electric service started on 24 September, initially with 6-car trains, later reduced to 4-car. The Metropolitan trains were soon modified to enclose the gated end and eventually to add sliding doors in the middle. Trains were increased to 5 cars in 1918 and the Metropolitan introduced new stock in 1921, with three pairs of sliding double doors on trailer cars. In 1926 the Metropolitan took over all inner circle workings except for three trains on Sundays.
On 1 July 1933 the Metropolitan and the District Railways were amalgamated with other Underground railways, tramway companies and bus operators to form the London Passenger Transport Board. Metropolitan Railway electric multiple units were refurbished in 1934 at Acton Works to become eighteen 5-car trains of Circle Stock, at first painted red and cream, later painted red all over to reduce costs. These trains included first-class accommodation, but this was downgraded in 1940. From 1947 these were replaced by 5-car trains of O and P Stock, with doors remotely operated by the guard, released by the transfer of F Stock to the Uxbridge line. The 1933 London Underground Beck map shows a Metropolitan line north of and stations and a District line south of these points. On the 1947 map the Metropolitan and District lines were shown together in the same colour and two years later in 1949 the Circle line was shown separately on the map.
In 1959–1960 Circle line trains were increased to 6 cars, the same length as those operating on the Hammersmith & City line, and the stock of the two lines was integrated with maintenance concentrated at Hammersmith depot, allowing Neasden depot to concentrate on the new A Stock. Aluminium C Stock trains, with public address systems and originally unpainted, replaced these trains from 1970. One person operation of the trains was proposed in 1972 but, due to conflict with the trade unions, was not introduced until 1984. In 2003, the infrastructure of the Circle line was partly privatised in a public–private partnership, managed by the Metronet consortium. Metronet went into administration in 2007 and the local government body Transport for London took over responsibilities.
On 7 July 2005, at about 08:50, bombs exploded on two Circle line trains. One was travelling between Liverpool Street and Aldgate and the other was at Edgware Road. The bombs killed 15 people, including the two suicide bombers. Following the attacks, the whole of the Circle line was closed until 8 August.
A day before a ban on drinking alcohol on public transport in London came into force, a party was held on 31 May 2008, mainly on the Circle line. Thousands of people attended and 17 were arrested by police due to disorderly behaviour, eventually causing several stations to be closed.
Prior to 13 December 2009, Circle line trains travelled in both directions around a simple loop with 27 stations and of track. In 2006 there were fourteen trains in service on the line with an interval between trains of  minutes during peak hours and 8 minutes off-peak; the minimum running time around the circle off-peak was  minutes, although timetabled stops at stations extended this.
In December 2009 the Circle line was extended to include the Hammersmith & City route from Edgware Road to Hammersmith. Rather than continuously running around the circle, trains now travel from Hammersmith to Edgware Road, generally going around the circle once before terminating at Edgware Road, and returning via the same route; occasionally, trains may also continue clockwise through Edgware Road to additional stations. The change was made to improve reliability and increase the service frequency on the Hammersmith branch.
The first S7 Stock train entered passenger service on the Hammersmith & City line on 6 July 2012, running a shuttle service between Hammersmith and Moorgate, and first ran on the Circle line on 2 September 2013. By June 2014 all services were provided by S7 Stock trains.
Hammersmith &amp; City line
The Hammersmith & City line is a line of the London Underground, colored salmon pink on the London Underground Map, running between Hammersmith in West London and Barking in East London.
Metropolitan line
The Metropolitan line is part of the London Underground. It is coloured in TfL's Corporate Magenta on the Tube map and in other branding. It was the first underground railway (or subway) in the world, opening on 10 January 1863. Today, part of that section are no longer served by the Metropolitan line, but by the Hammersmith & City, District and Circle lines. The main line runs from Aldgate in the City of London to Amersham, with branch lines to Uxbridge, Watford and Chesham. The line once went as far as Verney junction. After it was bought by the London Passenger Transport Board, the line was first cut back to Aylesbury, then to Amersham
The line runs parallel to Chiltern Railways London to Aylesbury line from Finchley Road to Amersham. Chiltern railways continues to run services to Aylesbury.
The Metropolitan Railway (also known as the Met) was a passenger and goods railway that served London from 1863 to 1933, its main line heading north-west from the City to what were to become the Middlesex suburbs. Its first line connected the mainline railway termini at , and King's Cross to the City, built beneath the New Road using the cut-and-cover method between Paddington and King's Cross, and in tunnel and cuttings beside Farringdon Road from King's Cross to near Smithfield. It opened on 10 January 1863 with gas-lit wooden carriages hauled by steam locomotives, the world's first underground railway. In 1871, when plans were presented for an underground railway in Paris, it was called the "Métropolitain" in imitation. The modern word metro is a short form of the French word.
The railway was soon extended from both ends and northwards via a branch from Baker Street. It reached Hammersmith in 1864 and Richmond in 1877, and completed the "Inner Circle" in 1884, but the most important route became the line north into the Middlesex countryside, where it stimulated the development of new suburbs. Harrow was reached in 1880, and the line extended as far as in Buckinghamshire, more than from Baker Street. From the end of the 19th century, the railway shared tracks with the Great Central Railway route out of Marylebone station.