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.
A table of the London Underground lines and some statistics can be found here.
Subsurface versus deep-level tube lines
Lines on the Underground can be classified into two types: subsurface (the earliest lines) and deep-level (those 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 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 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 actually an elevated railway running 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. 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 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. In contrast, 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, however, served by the London Overground's North London Line, and now also benefits 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 have long been outline plans to extend the Bakerloo line, which would provide services to Camberwell. Another large Tube-free area is Chelsea, a gap which the proposed Chelsea-Hackney Line would fill. The same line, if it is ever built, might also serve Clapham Junction, a large national rail interchange which is at present isolated from the tube network. In any case, both these areas are now served by the London Overground's West London Line.
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.