The Metropolitan Line is the longest sub-surface line of the London Underground, London's metro system. It was first opened in the 1860s as the Metropolitan Railway and was the world's first underground railway, the precursor to all modern subway systems. The line is also the origin of the modern word 'metro'. It is one of the Underground's four sub-surface lines, along with the Hammersmith and City Line, the Circle Line and the District Line. These are distinguished from the deep tube lines by the larger trains and tunnels and the use of the cut-and-cover method of tunnel construction.
The first section of the Metropolitan Railway ('Met' for short) opening in the 1860s along what is now part of the Circle Line in central London. From here it extended out into the countryside over the coming decades. By the turn of the 20th century, it was extending out into Buckinghamshire. As the railway extended, new housing developments were constructed along its route, marketed by the Met as 'metroland'. Such developments include Ruislip Manor and Northwood Hills (the latter oddly named considering it is largely on completely flat land). Originally the Metropolitan Line (renamed following the 1930s amalgamation of various railways to form the London Underground) also included the Hammersmith and City Line and the East London Line. The latter is now separate, taking with it the Aldgate-Barking leg of the Metropolitan, whilst the latter has now closed completely for conversion into part of the London Overground.
Starting from Aldgate in central London, the Metropolitan Line follows the Circle line as far as Baker Street station, where the Met has four platforms, separate from the two of the Circle Line. It then proceeds north towards Harrow on the Hill, first in tunnels before going overground and running together with the Jubilee Line, which then leaves at Wembley Park. After Harrow the Metropolitan main line continues north while a branch heads west to connect with the Piccadilly Line to Uxbridge. Returning to the Main Line, there are four tracks as far as Moor Park, with stopping Watford bound services using the inner tracks and fast Amersham bound trains using the outer lines. After Moor Park Watford trains take the branch to the east. This is a triangle junction, after which the station at Rickmansworth is reached.
Prior to 1961, Rickmansworth was the northern limit of electrification, and here trains switched to steam power. However, in 1961 the conductor rails were extended north to Amersham, with Met services being cut back to there from Aylesbury. At Chalfont and Latimer, the last station before Amersham, a short branch heads over to Chesham. This was originally intended to continue to Tring on the West Coast Main Line, but the Met abandoned this project in favour of extending further into Buckinghamshire. North of Amersham the tracks continue as part of the National Rail Network up to Aylesbury, and beyond that without passenger services to Quainton. Here the original Metropolitan Railway station survives in preservation.
The Metropolitan, like the entire London Underground, is electrified at 630V DC third and fourth rail. The trains used on the line are the 70mph capable A-stock trains (A for 'Amersham'). These directly replaced steam services on parts of the route when they were built in 1960 and 1962. They are due to be replaced in 2010 by the brand new S-stock trains, which will eventually operate on all of the sub-surface lines. Incidentally, the A-stock trains are some of the longest on the Underground, formed into eight-car sets. They are more like main line trains than any other Underground line's - being complete with forward-facing seating and luggage racks.