Track
The London Underground has always used the old     UK standard bullhead rail weighing 95 lbs/yd and wooden sleepers.   Only in the last     few years has there been a concerted effort to convert the track to flat bottom rail like the rest of the world.  The photo below shows the standard LU bullhead track which     can still be seen all over the system.  It also shows clearly the 3rd and 4th     insulated rails used for the LU traction current system.
Fig 1:  Photo of standard London Underground track at     Surrey Quays (East London Line)
Click on the image for the full size view     and description.
Current Rails
All London Underground Lines (including the W & C)     operate at 630 volts DC using third (positive) and fourth (negative) current rails.  The current rails are positioned so that the contact surface is higher than the running     rails.  This allows the collector shoes on the trains to pass over the running rails     without touching them.  The positive rail is 3 inches higher than the running rails,     while the negative rail is 1.5 inches higher.  The positive insulators are thus twice     the height of the negative ones and therefore have about twice the earth leakage     resistance, so the voltages are set with a proportional disparity between the positive and     negative voltage levels.  The positive rail is at a potential of 420 volts above     earth and the negative rail at 210 volts below earth.
In this view above of Surrey Quays (East London Line) the     arrangement of current rails is clearly seen.  The negative rail is mounted centrally between the two running rails.  The positive rail is mounted outside the running     rails.  Current rails are of a heavier cross section than running rails in this view.   Note how the positive rail is positioned away from the platform side of the track.   The standard position for the positive rail is usually on the left hand side in the     usual direction of travel.
Why 4 Rails?
The London Underground uses the four rail system for two main     reasons. Firstly, it was originally required by the government to limit the voltage drop     along the line to 7 volts.  This was intended to reduce problems caused by stray     currents causing electrolysis affecting utility pipes and cables.   Whilst this did     not affect the street tramways, whose vehicles were not heavy current users, the currents     drawn by trains could cause difficulties.  The solution was either to provide heavy     return cables and boosters or to use a fourth rail.  The fourth rail was chosen,     partly as a cheaper option and partly for signalling reasons.   As direct current     track circuits were to be used to control signals, an insulated return system for the     traction current was an effective way of separating the two systems.   Nowadays, this     is not so much of a problem as all track circuits are AC.
It is interesting to note that it is also possible to     maintain a service if one pole of the supply is earthed whereas, if a three-rail supply     system becomes earthed, the sub-station circuits breakers will detect the short circuit     (usually) and the power will be switched off.  On the Underground, if there is an     earth, it will be detected by special monitoring equipment and an alarm initiated in the     line control room.  The cause is then tracked down to, usually, a defect on a train,     a current rail problem or the lift/escalator supply, where this is still 630 volts     DC.  Modern trains on the Underground are provided with on-board earth fault     detectors.
It is important to ensure that positive and negative earths     do not occur in the same traction current section, as this can give rise to high current "loop circuits", which have been known to break down cable insulation and cause     fires.  See also Sectionalisation Gaps   below.
Curiously, the Bakerloo was opened (in March 1906) with the     polarity of the current rails reversed.  There were problems with leakage of the     positive rail into the cast iron tunnel segments so the positive side used the centre rail     and the negative used the outside rail.  This was exacerbated by the fact that the     line was supplied through the same system as the District.  The problem was solved on     the Piccadilly and Hampstead lines by concreting the lower part of the segments near the     track.  The Bakerloo was converted to normal in 1917, when the line's supply was     modernised.
Tube tunnels use a different cross section of current rails     as seen below.
Fig 2:  Photo of London Underground track at     a typical tube station with a suicide pit.
Click on the image for the full size view     and description.
Here, in this deep level tube station, the current rails are     of lighter section than the running rails, which have been upgraded to flat bottom     rail.  The white porcelain insulating "pots", which carry the current     rails, can also be seen.
In tube tunnels, the rails are mounted on small wooden blocks     set into concrete.  In the centre of the track, a drainage trough is provided and is     filled with ballast to reduce noise.  In stations, as shown above, a "suicide     pit" is provided between the running rails to assist with the removal of bodies from     under trains.   In the tunnels, a shallow pit is provided between the rails and is     filled with ballast to provide sound deadening and drainage.  Renewing track in     tunnels is a slow and difficult process due to the need to drill out the blocks and renew     the concrete.   It usually requires a speed restriction of 10 mi/h while work is in     progress.
Rail Greasing
Have you ever wondered what the screeching noise is that you     sometimes hear when a train goes round a curve?  It's the wheel flanges scraping     against the inside edge of the outer rail of the curve.  The best way to get rid of     the noise and reduce the wear on wheel and rail, is to lubricate the wheel flange.  This is done with either track mounted or train mounted lubricators.
Fig 3:  Track mounted flange greaser located at Rayners     Lane.
Click on the image for the full size view.
London Underground uses track mounted lubricators and the     photo above shows one type commonly seen around the system.  The problem with the     track mounted type is that is has to be refilled regularly and carefully adjusted to     ensure the correct amount of grease or oil is picked up by the wheel.s is not easy to     achieve.  One Tuesday morning some years ago, following a Public Holiday weekend,     almost the whole of the Victoria Line fleet was disabled with flatted wheels caused by     emergency braking at stations.  The trains had picked up too much grease from a badly     adjusted flange lubricator and had skidded at the following station trying to stop in the     right place.
Train mounted lubricators are usually in the form of a stick     of lubricant mounted on the bogie and applied to the wheel under light pressure.  They are easier to maintain than track lubricators and are generally favoured on new     railway systems.  Some recent LU stocks are fitted with these.
Traction Current     Supply System
Traction current is supplied to lines in     sections.  Sections can be anything from less than one kilometre to several     kilometres long.   Sections are separated by gaps in the current rails where the     sub-stations are located.  Each sub-station normally feeds the two sections on either     side of a gap.   The general arrangement of traction current supply on London     underground is shown in the diagram below.
Fig 6:  General arrangement of traction current supply on London Underground .
A section which is fed by a sub-station at each     end is known as double-end fed.  A section, usually towards a terminus, with a feed     at one end only is known as single-end fed.
At various places along the line, section     switches (Fig 6) are provided to allow small sections of line to be isolated from the     supply.  At a terminus, section isolating switches are provided to isolate the     platform sections from the supply, useful when necessary to deal with a power circuit     electrical fault on a train.  Section switches are usually manually operated on LU.   Many more modern systems allow remote control of section switches.
Fig 7:  A pair of unguarded section switches mounted on the tunnel     wall near Camden Town, Northern Line.  Photo by Donald McGarr.
Click on the image for the full size view.
Although traction current is intended primarily     for trains, there are some escalators still fed from the traction supply and a few     heavy-duty ventilation fans and lifts.  LU has to restrict the levels of current     regenerated by trains (where available) because, if it is too high, the additional voltage     will make the escalators run faster than they are supposed to and may cause trains to     reach too high a speed.
Sectionalisation Gaps
Sectionalisation gaps are similar to normal     substation gaps in that they separate the current rail feeds into a pair of adjacent     sections but the length of the gap is longer than the traditional gap.   This is so     that the shoes of a car cannot form a "bridge" between one section and the     next.  It avoids the risk of a train causing a section with current off being     temporarily made live from the adjacent section through the shoes and on-board cables of a     car creating a link.
Sectionalisation gaps are also arranged so that     the substation connections are entirely isolated from each other so that earth faults on     one section are not transmitted to the adjacent section.  Earth faults are caused     when one pole of the system becomes connected to earth, often because of an insulation     fault on a train or escalator, or because a current rail is linked to the ground with a     metal object.  If a positive earth fault occurs in the same area as a negative earth     fault some distance away, very high currents can be generated.   Originally, most of     the London Underground current supply system was linked through substations.  After a     couple of serious incidents in the late 1950s, where high fault currents generated fires     on board trains, the idea of sectionalisation was introduced and the system was de-coupled     at a number of strategic locations.  On lines built since that time (Victoria and     Jubilee extensions) sectionalisation has been provided at all substations.
Getting Gapped
One of the most embarrassing incident to happen     to a driver is to get his train "gapped".  This happens when the train     stops without any of its current collection shoes connected to the current rails.  The only two ways out of the problem is either to push the train onto the nearest current     rails or to make use of long cables (called gap leads) which can be used to connect a     socket on the train to shoes at the other ends of the leads which are then placed on the     rails.   Both procedures will cause a long delay and leave the driver with a lot of     explaining to do.
The worst locations for gapping are at the     entrances to depots.  There are always lots of breaks in the current rails because of     all the point and crossing work.  One of the most dangerous is at the south entrance     to Neasden Depot.    If you had a short train, it was best never to stop     and to hope that the shunter was available to give you the road number and stop point as     you ran in.  If he wasn't there, you just ran in and stopped where you knew it was     safe.   You could not risk stopping and gapping your train across both Jubilee tracks     and the southbound Metropolitan Line.
Tunnel Telephone     Wires
London Underground tunnels are fitted with     Tunnel Telephone Wires.  These are uncovered wires used to switch off traction     current in emergency.  A current of 5-10 volts runs through the wires and energises a     relay at the local substation(s).  If this current is lost, the relay is de-energised     and the circuit breaker opens at each end of that current section. This cuts off the 630v     traction current feed.  Even through the relay may then become re-energised, the     circuit breakers will remain open until reset. 
The relay can be de-energised through various     means such as the train operator pinching and rubbing the two telephone wires together or     the the operator connecting a portable handset to the wires.  This will then also     allow him to speak to the Line Controller.  Other connections on the wires in that     section include tunnel telephones and section plungers provided at stations.
The Line Controller has a remote facility (a     bank of switches on his desk for each traction current section on the line) which allows     him to effectively short circuit the tunnel telephone wires in the same way as the T/Op     shorting them.  Defective tunnel telephone wires/equipment will also result in the     relay becoming de-energised.  If the equipment cannot be reset due to a fault on the     tunnel telephone circuit, the circuit can be overridden.  A "T" board will     then be displayed to drivers at the relevant stations to tell the train operators that the     tunnel telephone wires are not working.
With the advent of train radio, the tunnel     telephone wires are rarely used to switch off traction current these days.  However,     they do offer an alternative means of contact to the Line Controller in an emergency if     there is a problem with the train radio.  Traction current will be switched off when     the handset is used but the Line Controller can soon arrange for it to be switched back on     again and using the tunnel telephone wires in this way will usually mean less of a delay     than trying to contact somebody by other available means.
Sometimes, too many trains in a section will     cause current to go off.  The more current drawn in a section, the more the traction     current voltage drops.  As a consequence, some equipment on a train may also cut, out     MGs/MAs for example.  Some black spot where this happens are between South Wimbledon     and Morden (Northern), Russell Square and Holloway Road and Wood Green to Southgate,     (Piccadilly Line) and Baker Street to Finchley Road (Met.). A favourite for South     Wimbledon to Morden in 1938 Stock days was when trains were queuing up to get into Morden     and it was nothing unusual to put the handle into motoring and find the tunnel lights come     on, put it back to off and the lights go out. Traction current wasn't lost, but the drop     in traction current voltage was sufficient that the tunnel lighting relays couldn't remain     energised and thus the lights came on.  From information posted by Roger Mather,     uk.transport.london, May 2000.
Traction     Current Sections
Listed below are the locations of the traction     current sections for some lines on the Underground.  Substation gaps (dividing the     two sections) are located where the feed from the substation goes to the current     rails.  Generally, the traction current section (or subsection as it is often known)     is named according to where it is fed from and the direction of travel, e.g. Stockwell to     Elephant & Castle NB.  Although not a substation location, Kennington CX branch     is divided up between Lambeth South and Lambeth North thus, Lambeth South-Kennington SB,     Kennington Loop, Kennington-Lambeth North NB.  Substations shown in capital letters     have Sectionalisation gaps.
Sectionalisation gaps were introduced in a     response to two serious incidents which occurred in 1958 and 1960, when earthing problems     caused trains fires on the Central Line at Redbridge and Notting Hill Gate.  The     cause was due to a positive earth on one part of the supply system and a negative earth on     another.  The resulting high current loop was sufficient to break down the insulation     on vulnerable parts of the trains in question.  To reduce the risk on a     re-occurrence, the power supply system was "sectionalised" into groups of     sub-stations and long gaps provided to separate them.  The long gaps prevents the     shoes on a train from bridging the gap and transferring a fault from one part of the     system to another.
Jubilee Line         Traction Current Sections
Stratford-Stratford Market depot