London Underground infrastructure
The railway infrastructure of the London Underground includes 11 lines, with 272 stations. There are two types of line on the London Underground: services that run on the sub-surface network just below the surface using larger trains, and the deep-level tube lines, that are mostly self-contained and use smaller trains. Most of the lines emerge on the surface outside the Central London area.
The oldest trains currently in service on the Underground are 1972 Stock trains on the Bakerloo line. The Underground is electrified using a four-rail system, the DC traction supply being independent of the running rails. Planned improvements include new stations, line extensions and more lines with automatic train operation.
Railway
The total length of railway on the London Underground is and made up of the sub-surface network and the deep-tube lines.In 1971/72 it was re-measured in kilometres using Ongar as the zero point.
Sub-surface network and deep-level tube lines
The Circle, District, Hammersmith & City, and Metropolitan lines are services that run on the sub-surface network, that has railway tunnels just below the surface and was built mostly using the cut-and-cover method. The tunnels and trains are of a similar size to those on British main lines. The Hammersmith & City and Circle lines share all their stations and most of the track with other lines. The Bakerloo, Central, Jubilee, Northern, Piccadilly, Victoria and Waterloo & City lines are deep-level tube lines, with smaller trains running through two circular tunnels with a diameter of about, lined with cast-iron or precast concrete rings, which were bored using a tunnelling shield. These were called the tube lines. Since the 1950s the term "tube" has come to be used to refer to the whole London Underground system.Many of the central London deep-tube line stations, such as those on the Central and Piccadilly lines, are higher than the running lines to help with deceleration when arriving and acceleration when departing. The deep-tube lines generally have the exclusive use of a pair of tracks. An exception is the Piccadilly line, which shares track with the District line between Acton Town and North Ealing and with the Metropolitan line between Rayners Lane and Uxbridge. The Bakerloo line shares track with London Overground Lioness line between Queen's Park and Harrow & Wealdstone.
There are of cut-and-cover tunnel and of tube tunnel, the other 55% of the system running above ground. Trains generally run on the left-hand track, although in some places, for example the Central line east of St Paul's station, tunnels are dug one above each other. The Victoria line has right-hand running between Warren Street and King's Cross St Pancras, allowing cross-platform interchange with the Northern line between northbound and southbound trains at Euston.
Six of the 32 London boroughs are not served by the Underground. All of these are south of the River Thames: Bexley, Bromley, Croydon, Kingston, Lewisham and Sutton. The borough of Lewisham used to be served by the Underground on the East London line – now part of London Overground – at New Cross and New Cross Gate. Of the boroughs through which the Underground lines pass, Hackney is served solely by Manor House station on the Piccadilly line on the very north-western boundary of the borough. For the most part however, some of the tube lines like the Central line skirt around the perimeter of Hackney rather than going straight through it. Richmond upon Thames is served only by Kew Gardens and Richmond stations on the District line, which shares its tracks with London Overground's Mildmay line between Gunnersbury and Richmond. The Royal Borough of Greenwich had no Underground station until North Greenwich station opened in 1999 on the Jubilee line extension.
Electrification
The lines are electrified with a four-rail DC system. The configuration and potential of the conductor rails varies across the network. As of 2020, there are three different conductor rail configurations:- The original configuration is nominally 630 V with a −210 V centre conductor rail and a +420 V outside conductor rail. This is the default configuration wherever deep-level tube trains operate.
- Beginning in 2016, sections of the sub-surface network were reconfigured to a 750 V configuration. As of 2019, the entire sub-surface network uses this configuration except for Uxbridge to Finchley Road as 1973 Stock and 1996 Stock share the railway.
- In areas where Underground and third rail rolling stock share tracks, the centre conductor rail is electrically connected to the running rails. This still results in a 750 V voltage, but in a 0 V/+750 V configuration. Lines configured in this manner include:
- * Between Gunnersbury and Richmond on the District line, shared with London Overground
- * Between East Putney and Wimbledon on a section of the District line that was transferred from British Rail
- * Between Queen's Park and Harrow & Wealdstone on the Watford DC line, served by Bakerloo line and London Overground trains
The traction current has no direct earth point, but there are two resistors connected in series across the traction supply. The positive resistor is twice as great as the negative resistor, since the positive rail carries twice the voltage of the negative rail. The junction point of the resistors is earthed, establishing the reference point between the positive and negative rails by voltage division. The resistors are great enough to prevent large currents flowing through the earthed infrastructure.
Ventilation and cooling
The Metropolitan Railway's first line was built in the belief that it would be operated by smokeless locomotives and with little thought given to ventilation. Initially the smoke-filled stations and carriages did not deter passengers, the ventilation being later improved by making an opening in the tunnel between King's Cross and Gower Street and removing glazing in the station roofs, and the later extensions and the District Railway were built with stations in the open. With the problem on the original line continuing after the 1880s, conflict arose between the Met, who wished to make more openings in the tunnels, and the local authorities, who argued that these would frighten horses and reduce property values. This led to an 1897 Board of Trade report that reported a pharmacist was treating people in distress after having travelled on the railway with his 'Metropolitan Mixture'. The report recommended more openings be authorised but the underground sections of the Metropolitan and District railways were electrified before these were built.Forced ventilation was not considered when the deep-tube Central London Railway opened in 1900, engineers considering that the movement of the electric trains would give sufficient air circulation. However, soon after opening there were complaints about a smell that the company couldn't explain, and by 1911 they had installed a system of fans injecting filtered air and ozone. Exhaust fans had been fitted at most stations when the Underground Electric Railways Company opened its three tube lines in 1906–7, a maximum temperature of in hot weather being advertised on the Bakerloo line. However, over time heat from the trains has warmed up the tube tunnels, and in 1938 approval was given to a £500,000 programme to improve the ventilation and an experimental refrigeration plant was installed in a lift shaft at Tottenham Court Road. More recently, temperatures of were reported in the 2006 European heat wave. It was reported in 2002 that, if animals were being transported, temperatures on the Tube would break European Commission animal welfare laws. A 2003 study stated that air quality was seventy-three times worse than at street level, with twenty minutes on the Northern line having "the same effect as smoking a cigarette".
The main purpose of the London Underground's ventilation fans is to extract hot air from the tunnels, a system on the Jubilee line extension being designed to allow cooling of the tubes at night. Fans over the network are being refurbished, although complaints of noise from local residents preclude their use at full power at night. Following a successful demonstration of a heat pump in 2001, funds were given to the School of Engineering at London's London South Bank University to develop a prototype; work began in April 2002. A prize of £100,000 was offered by the Mayor of London during the hot summer of 2003 for a solution to the problem, but the competition ended in 2005 without a winner. A year-long trial of a groundwater cooling system began in June 2006 at Victoria station. The University's system comprised three fan coil units that use water that has seeped into the tunnels and is pumped from the tunnels to absorb the heat after which it is discharged in the sewer system. The scheme was one of the winners in the Carbon Trust's 2007 Innovation Awards. In 2012 air cooling units were installed on platforms at Green Park station using cool deep ground water and at Oxford Circus using chiller units at the top of an adjacent building. New air-conditioned trains have been introduced on the sub-surface lines, but was initially ruled out for the tube trains due to space being considered limited on the tube trains for air-conditioning units and that these would heat the tunnels even more. The New Tube for London, which will replace the existing fleet of the Piccadilly, Bakerloo, Central and Waterloo & City lines, is planned to have air-conditioning for the tube trains along with better energy conservation and regenerative braking.