London Underground infrastructure

Part of a series of articles on The Tube
Overview History Timeline Infrastructure Stations Trains Popular Culture Map London Transport portal

The infrastructure of the London Underground includes 11 lines, with 268 stations. Lines on the Underground can be classified into two types, subsurface and deep-level or tube. 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

1996 Stock trains at Stratford Market Depot

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 train are currently being introduced: 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, and the first S stock unit entered service in 2010.

In addition to the passenger stock described above, there is engineering stock such as ballast trains and brake vans. These are identified by a 1-3 letter prefix, then a number.

Stations

The Underground serves 270 stations. 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.

Zone 1 (central zone) of the Underground (and DLR) network in a more geographically accurate layout than the usual Tube map, using the same style

The longest distance between two stations is 6.26 km (3.89 mi) between Chalfont & Latimer and Chesham on the Metropolitan Line. The longest distance between two underground stations is between Seven Sisters and Finsbury Park on the Victoria Line. The shortest distance between adjacent stations is the 260 metres (280 yd) between the Leicester Square and Covent Garden 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 east is Upminster on the District Line. Chesham on the Metropolitan Line is both the northernmost and westernmost station on the network. 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. The furthest station from Central London 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 mi) from Charing Cross.

Lines

The table below lists each line, the colour used to represent it 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

Underground trains come in two sizes, larger subsurface trains and smaller tube trains. A Metropolitan line A Stock train (left) passes a Piccadilly line 1973 Stock train (right) in the siding at Rayners Lane

Example of a deep-level (tube) tunnel. This is the Jubilee Line, near Green Park, during the 3 April 2008 power failure.

Lines on the Underground can be classified into two types: subsurface (the earliest lines) and deep-level or tube (built since about 1890). It is the latter that are strictly the "tube" lines, although in more recent times the term "tube" has come to be used loosely to refer to the whole London Underground system. 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 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 below the surface, at 58.5 metres (64 yards, 192 feet). However, it is not the deepest below sea level, as its 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). The highest station is Amersham on the Metropolitan line.

Lines of both types usually emerge on to 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's route mileage is in tunnel: despite its name, substantial parts of the system are on brick viaducts. 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, 18 metres (60 feet) above the ground.^[1]

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 possible 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, the London, Brighton and South Coast Railway, and the South Eastern and Chatham Railway, then being electrified, reducing 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. Suburban traffic obstructed their long-distance operations and required substantial infrastructure investment without providing compensating returns.

The seventh unserved borough is Hackney, although Manor House and Old Street stations lie just outside its boundary. The borough is served by the London Overground's North London Line and 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 have long been outline plans to extend the Bakerloo line to Camberwell. Another large Tube-free area is Chelsea, a gap which the proposed Chelsea-Hackney Line would fill. The same line might also serve Clapham Junction, a large National Rail interchange which is isolated from the tube network. Both these areas are now served by the London Overground's West London Line.

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 trains to mainland Europe via the Channel Tunnel.

Electrification

Canary Wharf tube station (Jubilee line)

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. A top-contact third rail beside the track is energised at +420 V DC, and a top-contact fourth rail centrally between the running rails is at -210 V DC, combining to provide the traction voltage of 630 V DC.

Most tube lines run in cast-iron tunnels, with some of the more recent constructions using concrete lining. Using a third-rail scheme necessitates that the return current is conducted through an 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 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 to permit through running with the tube lines, there being no 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 that 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 at +630 volt DC.

Planned improvements

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 precise timescale has been announced.^[2]

This section may need to be updated. Please update this section to reflect recent events or newly available information, and remove this template when finished. Please see the talk page for more information. (March 2012)

• 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.^[3]

Line upgrades

Each London Underground 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. This project was due for completion in 2009, but has been subject to persistent delays, largely caused by unforeseen difficulties in testing the new software while still running trains under the old signalling.^[4] When this work is complete, a similar upgrade will be performed on the Northern line, originally scheduled for completion in 2012 and now expected to be finished in 2014.^[4] Both lines already have modern rolling stock.
• The Victoria line has been receiving new 2009 Stock trains since 2009. They have a higher capacity and 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 1min 40secs), with a total capacity increase of 21%.^[5]
• All the subsurface routes -- Metropolitan line, Hammersmith & City line, Circle line and District line -- are having their signalling systems renewed, replacing unreliable, life-expired installations which in some cases date from before the Second World War.^[5] In addition, these lines are receiving new S Stock trains, which are being introduced in phases from 2009 to 2015.^[6] The new trains feature inter-car gangways enhancing passenger safety, regenerative braking leading to a 20-25% reduction in carbon dioxide emissions, air conditioning and improved acceleration and braking allowing an increase in train frequency. The first S stock trains began work on the Metropolitan line in summer 2010.^[5] 75 D78 trains on the District line are scheduled to be replaced by 2015. Lines currently served by six-car trains (Hammersmith & City and Circle) will get seven-car trains once the necessary platform-lengthening works are completed. This, together with the new signalling, will increase capacity on those two lines by 65%.^[5]
• The Piccadilly line and Bakerloo line will receive signalling upgrades by 2015^[4] and 2020 respectively. LUL is developing a concept (called "Evo") for a new generation of tube train for these lines, but this is only at the early planning stage.^[5]
• Upgrade programmes on the Waterloo & City line (without ATO) and Central line are 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.^[7] Conventional air conditioning has been ruled out on existing rolling-stock on the deep tube lines because of the lack of space for equipment on the trains and the problems of dispersing the waste heat this would generate. It is hoped that a new generation of lightweight low-energy tube trains will solve this problem in the future.^[5] Meanwhile trials of a groundwater cooling system have been held at Victoria station. The Underground also advises passengers to carry a bottle of water to help keep cool. Waste heat disperses better in the subsurface tunnels and the new S Stock trains do have air-conditioning.^[8]

• On 15 March 2007 it was announced that there would be a trial of mobile phone coverage on the Waterloo & City line.^[9] 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.

References