London Underground infrastructure

The infrastructure of the London Underground includes 11 lines, which serve 268 stations by rail. Lines on the Underground can be classified into two types: subsurface and deep-level. Lines of both types usually emerge onto the surface outside the central area. While the tube lines are for the most part self-contained, the subsurface lines are part of an interconnected network. The Underground uses rolling stock built between 1960 and 2005. The Underground is one of the few networks in the world that uses a four-rail system. The additional rail carries the electrical return that on third-rail and overhead networks is provided by the running rails. Planned improvements include new stations, line extensions, computerised signalling, automatic train operation (ATO), track replacement, new rolling stock, new cooling systems, and mobile phone coverage underground.

Rolling stock
The Underground uses rolling stock built between 1960 and 2005. Stock on subsurface lines is identified by a letter (such as A Stock, used on the Metropolitan line), while tube stock is identified by the year in which it was designed (for example, 1996 Stock, used on the Jubilee line). All lines are worked by a single type of stock except the District line, which uses both C and D Stock. Two types of stock are currently being developed &mdash; 2009 Stock for the Victoria line and S stock for the subsurface lines, with the Metropolitan line A Stock being replaced first. The introduction of new stock for the Victoria line began in 2009, with the first S stock unit entering service in 2010.

In addition to the Electric-Multiple units described above, there are Engineering Stock, such as ballast trains and brake vans. They are identified by a 1-3 letter prefix, then a number.

Stations
The Underground serves 268 stations by rail. An additional seven locations are served by the replacement bus services for the East London line that will be offered until the East London Line Extension opens as part of the London Overground in 2010. Fourteen Underground stations are outside Greater London, of which five (Amersham, Chalfont & Latimer, Chesham, and Chorleywood on the Metropolitan Line, and Epping on the Central Line) are beyond the M25 London Orbital motorway.

The longest distance between two stations is 6.26 km (3.89 miles), between Chalfont & Latimer and Chesham on the Metropolitan Line. The longest distance between two underground stations is between the Seven Sisters and Finsbury Park stations on the Victoria Line. The shortest distance between adjacent stations – by rail - is the 260 m between the Leicester Square and Covent Garden stations on the Piccadilly Line.

The shortest distance between two stations is between Charing Cross and Embankment, a distance of 100m (109 yards), which is stated on many tube maps.

The station furthest south is Morden on the Northern Line. The station furthest north is Chesham on the Metropolitan Line. The station furthest east is Upminster on the District Line. Using Charing Cross as the marker, Chesham has been the furthest station from Central London since 1994, prior to which it was the now closed Ongar station. The furthest station from Central London ever regularly served by a London Underground service was Verney Junction which was at the (then) far end of what is now the Metropolitan Line. Located in a hamlet in rural Buckinghamshire it is over 64 km (40 miles) from Charing Cross.

Lines
The table below lists each line; the colour used to represent each on Tube maps, the date the line became operational and the first section opened (not necessarily under the current line name), the date the line gained its current name (in some cases originally with the word "Railway" rather than "line"), and the type of tunnel used in the central area.

Until 2007 there was a twelfth line, the East London line, but was transferred to the London Overground network in May 2010, following the opening of the extensions.

Subsurface versus deep-level tube lines


Lines on the Underground can be classified into two types: subsurface and deep-level. The subsurface lines were dug by the cut-and-cover method, with the tracks running about 5 m below the surface. Trains on the subsurface lines slightly exceed the standard British loading gauge. The deep-level or tube lines, bored using a tunnelling shield, run about 20 m below the surface (although this varies considerably), with each track in a separate tunnel lined with cast-iron or precast concrete rings. These tunnels can have a diameter as small as 3.56 m (11 ft 8.25 in) and the loading gauge is thus considerably smaller than on the subsurface lines. Hampstead is the deepest station under the surface at 58.5 metres (64.0 yards, 192 feet). However, it is not the deepest below sea level as the station's surface building is near the top of a hill. The deepest platforms below sea level are the Jubilee Line platforms at Westminster which are -32 metres (-35 yards).

Lines of both types usually emerge onto the surface outside the central area, except the Victoria line, which is in tunnel except for its depot, and the very short Waterloo & City line, which runs entirely in the central area and has no surface section. Only 45% of the Underground is in tunnel. The highest point above ground on the Underground is the Dollis Brook Viaduct over Dollis Road between Finchley Central and Mill Hill East on the Northern Line. It is 18 metres (60 feet) above the ground.

While the tube lines are for the most part self-contained, the subsurface lines are part of an interconnected network: Each shares track with at least two other lines. The subsurface arrangement is somewhat similar to the New York City Subway, which also runs separate "lines" over shared tracks.

Non-served areas
Seven of the 32 London boroughs are not served by the Underground. Six of these are south of the River Thames: Bexley, Bromley, Croydon, Kingston, Lewisham and Sutton. This lack of lines and stations is sometimes attributed to the geology of that area, the region being almost one large aquifer. Another reason is that during the great period of tube-building in the early 20th century south London was already well served by the efficiently-run suburban lines of the London and South Western Railway, London, Brighton and South Coast Railway and the South Eastern and Chatham Railway, then being electrified, which obviated the need for Underground expansion into those areas. Suburban traffic was essential to the viability of the southern railways, while railways to the north and west were able to focus on long-distance traffic, which was profitable and was not subject to the short-term traffic peaks of suburban traffic. In contrast, suburban traffic obstructed their long-distance operations and required substantial infrastructure investment, without providing compensating returns.

Lewisham was served by East London Line stations at New Cross and New Cross Gate until 2007. The former Tube service will be replaced by the London Overground metro-style railway from 2010, which will incorporate an extension to West Croydon and Crystal Palace, passing right through the borough.

The seventh unserved borough is Hackney, although Manor House and Old Street stations lie just outside its boundary. The borough is also served by London Overground's North London Line, and will benefit from its East London Line extension.

Greenwich borough was unserved until North Greenwich station opened on the Jubilee line extension in 1999.

Parts of Inner London not served by the London Underground or national rail include a large section at Camberwell and Walworth, although there has been some mentions of plans to extend the Bakerloo line which would provide services to Camberwell. Another large area is Chelsea, which the proposed Chelsea-Hackney Line intends to fill. The same proposal may also possibly be serving Clapham Junction, a large national rail interchange which is at present isolated from the tube network.

International connections
The Underground serves London Heathrow Airport for national and international flights; and St Pancras International (via King's Cross St. Pancras) for Eurostar services to mainland Europe, via the Channel Tunnel.

Electrification
The Underground is one of the few networks in the world that uses a four-rail system. The additional rail carries the electrical return that on third-rail and overhead networks is provided by the running rails. On the Underground a top-contact third rail is beside the track, energised at +420 V DC, and a top-contact fourth rail is centrally between the running rails, at -210 V DC, which combine to provide a traction voltage of 630 V DC.

Most tube lines run in cast-iron tunnels (only some of the more recent constructions use concrete tunnel lining). Using a third-rail scheme necessitates that the return current is conducted through one (earthed) running rail. Such current is just as easily able to travel through the cast-iron tunnel lining, and unless the joints between the sections are electrically sound, the current will arc across the sections causing considerable damage, or corrode the tunnel segments via electrolysis. There are also many cast-iron gas and water mains in the vicinity of the tube tunnels, and the return current would travel along these just as easily. Some of these mains date back to the 19th century and the joints between separate sections would certainly not have been designed to be electrically sound, as deep-level electric tube trains were some way off.

Another advantage of the fourth rail system is that the two running rails are available exclusively for track circuits, of which there are many.

The surface sections of the lines are constructed using fourth-rail purely to permit through running with the tube lines, there being no other technical reason to do so.

The traction current has no direct earth point, but there are two resistors connected across the traction supply. The centre tap of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are large enough to prevent large currents flowing through the earthed infrastructure. The positive resistor is twice as large as the negative resistor, since the positive rail carries twice the voltage of the negative rail.

Some above-ground sections are shared with National Rail trains which use the three-rail system. On these sections the fourth rail is bonded to the running rails, to keep it at earth potential, and the third rail is held at +630 volt DC.

Extensions and new stations

 * The Bakerloo line may be re-extended to Watford Junction from its current terminus at Harrow & Wealdstone. This is part of the Transport 2025 strategy, but no more precise timescale has been announced.


 * TfL and Hertfordshire County Council would like to connect the Watford branch of the Metropolitan line to the disused Croxley Green Network Rail branch, between Croxley Green and Watford West. A new station would be built at Ascot Road as a replacement for Croxley Green, Watford West would be heavily refurbished and Watford (Metropolitan) would close. This extension would bring the Underground back to central Watford and the important Watford Junction main-line station. The current timetable suggests a 2011 opening date, but the extension currently lacks funding and planning permission.

Line upgrades
Each line is being upgraded to improve capacity and reliability, with new computerised signalling, automatic train operation (ATO), track replacement and station refurbishment, and, where needed, new rolling stock.


 * During 2007, work began to install moving block signalling and ATO on the Jubilee line, for completion in 2009. When this work is complete, a similar upgrade will be performed on the Northern line, for completion in 2012. Both lines already have modern rolling stock.
 * The Victoria line will receive new 2009 Stock trains from 2009 onwards. They will be higher in capacity and offer improved acceleration. A new ATO system will be brought into service once the old fleet has been withdrawn. When all upgrades are complete in 2013, train frequency will have improved from 28 trains per hour to 33 (averaging around a train every 1min40secs).
 * The Metropolitan line, Hammersmith & City line, Circle line and District line will receive new S Stock trains, to be introduced in phases from 2009 to 2015. New trains will feature inter-car gangways enhancing passenger safety, regenerative braking leading to a 20-25% reduction in carbon dioxide emissions, and improved acceleration and braking allowing an increase in train frequency. The last trains to be replaced, 75 District line trains, are currently receiving interim refurbishment. Lines that are currently served by six-car trains will get seven-car trains, once necessary platform-lengthening works are completed.
 * The Piccadilly line and Bakerloo line will receive new rolling stock and other upgrades by 2014 and 2020 respectively.

Upgrade programmes on the Waterloo & City line (without ATO) and Central line are largely complete.

Other projects

 * Shepherd's Bush tube station on the Central line has been completely rebuilt both above and below ground.
 * Victoria and King's Cross St Pancras stations will have new passageways and an extra ticket hall each to improve capacity.
 * In summer, temperatures on parts of the Underground can become very uncomfortable due to its deep, narrow and poorly ventilated tube tunnels: temperatures of 47 °C (117 °F)  were reported in the 2006 European heat wave. Conventional air conditioning has been ruled out on the deep lines because of the lack of space for equipment on trains and the problems of dispersing the waste heat this would generate. A year-long trial of a groundwater cooling system began in June 2006 at Victoria station. If successful, the trial will be extended to 30 other deep-level stations. The Underground also advises passengers to carry a bottle of water to help keep cool. Waste heat disperses better in the subsurface tunnels and S Stock trains will have air-conditioning.


 * On March 15, 2007 it was announced that there will be a trial of mobile phone coverage on the Waterloo & City line. At the earliest, the trial will start in April 2008, when coverage will be available on the platforms at Waterloo and Bank stations. After this, coverage will be extended to the tunnel between the two stations. The trial will look at the viability of extending coverage across the rest of the Underground network.
 * Although not part of London Underground, the Crossrail scheme will provide a new route across central London integrated with the tube network.