third rail: Definition and Much More from Answers.com

A third rail is a method of providing electricity to power a railway by means of a continuous rigid conductor mounted alongside the railway track or between the rails. It is used typically in a mass transit or rapid transit system, which has alignments in own corridors, fully or almost fully segregated from the outside environment. A list of lines or networks equipped with a third rail is provided further below. Third rail systems generally supply direct current to power the trains.

The third rail system of electrification is unrelated to the third rail used in dual-gauge railways.

History

Third-rail electric systems are, apart from on-board batteries, the oldest means of supplying electric power to trains on railways using own corridors, particularly in cities. Overhead power supply was initially almost exclusively used on tramway-like railways, though it also appeared slowly on mainline systems. (This statement describes the general trend; early particular cases may have been different.)

An experimental electric train using this method of power supply was developed by the German firm of Siemens & Halske and shown at the Berlin Industrial Exhibition of 1879. This pioneer electric railway had its third rail placed between running rails. At some early electric railways, though, one of the running rails could be the current conductor, as was the case of the 1883-opened Volk's Electric Railway in Brighton. Soon it was given an additional power rail in 1886 (the railway is still operating). The Giant's Causeway Tramway followed, equipped with an elevated outside third rail in 1883 (but later converted to overhead wire pickup). The first railway to use the central third rail was the Bessbrook & Newry Tramway, opened in Ireland in 1885 but now, like the Giant's Causeway line, closed. Also in the 1880s third-rail systems began to be used in public urban transport. Trams were first to benefit from it, but they used conductors built in conduit below the road surface (cf. Conduit current collection), and usually on selected parts of the networks. This was first tried in Cleveland (1884) and in Denver (1885) and later spread to many big tram networks (e.g. Manhattan, Chicago, Washington DC, London, Paris - all closed).

A third rail supplied power to the world's first electric underground railway, the City & South London Railway, which opened in 1890 (now part of the Northern Line of the London Underground). In 1893 the world's second third-rail powered city railway opened in Britain - the Liverpool Overhead Railway (closed 1956 and dismantled). The first US third-rail powered city railway in revenue use was the 1895-opened Metropolitan West Side Elevated, which soon became part of the Chicago 'L'. In 1901, Granville Woods, a prominent African-American inventor, was granted a U.S. Patent  , covering various proposed improvements to third rail systems. This has been cited to claim that he invented the third rail system of current distribution. However, by that time there had been numerous other patents for electrified third-rail systems, including Thomas Edison's U.S. Patent   of 1882, and third rails had been in successful use for over a decade, in installations including the rest of Chicago 'elevateds', as well as these in Brooklyn, New York (if not to mention the development outside the US). To what extent Woods' ideas were adopted is thus a matter of controversy.[1]

In Paris, in 1900, third rail appeared in the mainline tunnel connecting the Gare d'Orsay to the rest of the CF Paris-Orléans network. Mainline third rail electrification was later expanded to some suburban services in the French capital.

Top contact third rail (cf. below) seems to be the oldest form of power collection. Railways pioneering in using other, less hazardous types of third rail, were the New York Central Railroad on the approach to its NYC's Grand Central Terminal (1907 - another case of a third-rail mainline electrification) and the Hochbahn in Hamburg (1912) - both had bottom contact rail. However, the Manchester-Bury Line of the Lancashire & Yorkshire Railway tried the side contact rail (1917). These technologies appeared in wider use only at the turn of the 1920s and in the 1930s at, e.g., large-profile lines of the Berlin U-Bahn, the Berlin S-Bahn and the Moscow Metro.

In 1956 world's first rubber-tyred railway line was opened. This was Line 11 of Paris Metro. Power rail evolved into a pair of guiding rails required to keep the bogie in proper position on the new type of track. This solution was modified on the 1971-opened Namboku Line of Sapporo Subway, where a centrally placed guiding/return rail was used plus one power rail placed laterally as usually on steel rail railways (cf. photo).

The third rail technology at street tram lines has recently been revived in the new system of Bordeaux (2004). This is a completely new technology (cf. below).

Third rail, being the older of the two electric current supply methods, is by no means obsolete. There are, however, countries (particularly Japan, South Korea, India, Spain) more eager to adopt overhead wiring to their urban railways. But in the same time there were (and still are) many new third rail systems built elsewhere, including technologically advanced countries (i.e. Copenhagen Metro, Taipei Metro, Wuhan Metro). Bottom powered railways (it may be too specific to use the term 'third rail') are also usually these having rubber-tyred trains, no matter if it is a heavy metro (except two other lines of Sapporo Subway) or a small capacity people mover (PM). Practically the only type of railways where third rail is no longer used in new systems is regional and long distance rail, which require higher speeds and voltages.

Technical aspects

The third rail is usually located outside of the two running rails, but occasionally runs between them. The electricity is transmitted to the train by means of a sliding "shoe" (pick-up or contact shoe) which is held in contact with the rail. On many systems an insulating cover is provided above the third rail to protect employees working near the track; sometimes the shoe is designed to contact the side (called side running) or bottom (called bottom running) of the third rail, allowing the protective cover to be mounted directly to its top surface. When the shoe slides on top, it is referred to as "top running". When the shoe slides on the bottom it is not affected by the build-up of snow or leaves.

As with overhead wires, the return current on a third-rail system usually flows through one or both running rails, and leakage to ground is not considered serious. Where trains run on rubber tires, as on parts of the Paris Métro, Mexico City Metro and Santiago Metro, as well as on all of the Montreal Métro, live guide bars must be provided to feed the current. The return is effected through the rails of the conventional track between these guide bars (see rubber-tired metro). Another design, with a third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails), is used by a few steel-wheel systems, see fourth rail. The London Underground is the largest of these, see railway electrification in Great Britain.

In line M1 of the Milan underground, the third rail is used as the return electrical line (with potential near the ground) and the live electrical connection is made with a sliding block on the side of the car contacting an electrical bar located next to the railway (between the railway and the opposite direction railway) approximately 1 m (3') above the rail level. In this manner there are four rails. In the northern part of the line the more common overhead lines system is used.

The third rail is an alternative to electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. Whereas overhead-wire systems can operate at 25 kV or more, using alternating current (AC), the smaller clearance around a live rail imposes a maximum of about 1200 V (Hamburg S-Bahn), and direct current (DC) is used. Trains on some lines or networks use both power supply modes (cf. below, "Compromise systems").

One method for reducing current losses (and thus increase the spacing of feeder/sub stations - a major cost in third rail electrification) is to construct the conductor rail of a hybrid aluminium/steel design (or composite conductor rail). The aluminium, which is a better conductor of electricity, combined with a running face of stainless steel, which gives better wear, aims to match the existing steel conductor rails.

There are currently several marketed ways of attaching the stainless steel to the aluminium. The oldest is a co-extruded method, where the stainless steel is extruded with the aluminium. This method has suffered, in isolated cases, from de-lamination (where the stainless steel separates from the aluminium); this is said to have been eliminated in the latest co-extruded rails. A second method is an aluminium core, upon which two stainless steel sections are fitted as a cap and linear welded along the centre line of the rail. Because aluminium has a higher coefficient of thermal expansion than steel, the aluminium and steel must be positively locked to provide a good current collection interface. A third method rivets aluminum bus strips to the web of the steel rail. The photo on the right depicts such a rail.

Advantages of third rail

Cost

Third-rail systems are cheaper to install than overhead wire systems, less prone to weather damage (other than flooding and icing, which cause major problems), and better able to fit into areas of reduced vertical clearance, such as tunnels and bridges. In many countries they were perceived as key means of reducing construction costs of tunnels, hence their popularity at underground railways.

Visual appeal

Third-rail systems cause less visual intrusion: they do not need overhead lines, which some people perceive as unsightly. Singapore, for example, has banned overhead wires on lines outside tunnels. Urban street railways have been built, for example in Washington DC, London, and Brussels, that carry the conductor rail within a slotted box in the center of the track (conduit current collection), primarily to avoid unsightly overhead wires and poles. These resemble the cable slot for a street cable car as seen in San Francisco. Rather than a mechanical grip, an insulated electrical pickup extends into the slot.

Robustness

Third-rail systems are more robust than overhead line systems, as the conductor rail is able to take higher mechanical forces than the contact wire of an overhead line system. The shoegear on a train is designed to shear off if it hits the conductor rail too hard, but as a train has many sets of shoegear, it is able to continue its journey. By contrast a pantograph is more likely to get tangled up in the overhead wires and not be able to continue its journey.

Maintenance access

Because it lies near the ground within easy reach, instead of many feet up in the air, a third rail system allows easy maintenance.

Compatibility

Many railways use third rail and DC power, even where overhead lines would otherwise be practical, due to the high cost of retrofitting. Every expansion of such system must cope with the problem of compatibility. It usually leads for the choice of already existing technology.

Disadvantages of third rail

Third-rail systems have a number of problems and disadvantages, including:

Safety

An unguarded electrified rail is a safety hazard, and some people have been killed by touching the rail or by stepping on it while attempting to cross the tracks. However, such incidents are usually the result of carelessness on the part of the victim. The principal hazard is probably associated with level crossings. While their number on third rail lines is normally reduced to none, they still occur at some systems, particularly on rural and suburban portions of the network. One notable example of a Metro line running a third rail at ground level is the outer ends of the present Brown Line and Pink Line of the Chicago 'L', running on street level in a densely populated neighborhood. The conductor is discontinued in the level crossing area. Pedestrians may be discouraged from trespassing into railway area by means of perforated panels difficult to step on (cattle-cum-trespass guards). They are laid between rails alongside the road.

Intercity ground level third rail systems are the norm in the southeast of England, level crossings are handled in a fashion similar to the Chicago system. A few interurban electric railways attempted to utilize third rail in the USA, these were quickly abandoned as impractical outside of New York City commuter lines such as the Long Island Rail Road. Both the US and UK intercity systems address safety through extensive fencing and warning signage.

There are urban legends that people have died while urinating on the third rail (the urine stream supposedly completes an electrical circuit that electrocutes the victim); a non-continuous stream has been demonstrated by MythBusters to be unable to conduct electricity [2]. This myth may be partially perpetuated by a 1977 incident that occurred in Chicago where an intoxicated pedestrian suffered a fatal electrocution injury while trespassing to urinate on the grade-level CTA Brown Line right-of-way near Kedzie Avenue. However, the death occurred as a result of the passenger making physical contact with the third rail (not as a result of an electrical circuit being completed via his urine stream)[3].

• A new tramway system in Bordeaux, France surmounts the safety problem by using a third rail divided into insulated segments only a few metres long. Each segment is live only while completely covered by a tram, so there is no risk of a person or animal coming into contact with a live rail (see Third-rail power for trams and stud contact electrification for more information). This system would not be suitable for higher speeds, and the cost of breaking the live rail into short sections is considerable. This system was developed mainly for aesthetic reasons, to avoid overhead wires in front of the town hall.

• Other safety precautions can be made to reduce the risk of the third rail. Many subway systems, such as the BART and the Washington Metro, use sturdy sheaths to cover its third rails and always place the rail on the further side of the track away from where passengers would normally be. If someone falls on the tracks, there is room to return safely to the platform (or crawl under the platform) without the danger of stepping on the third rail.

Limited capacity

A relatively low voltage is necessary in a third-rail system — otherwise, electricity would arc from the rail to the ground or the running rails — but the resulting higher current (sometimes upwards of 3,000 amperes) causes more proportional voltage drop per mile, meaning that electrical feeder sub-stations have to be set up at frequent intervals along the line (generally no more than 10 miles or 16 km apart), increasing operating costs. The low voltage also means that the system is prone to overload, which makes such systems unsuitable for freight or high-speed trains demanding high amounts of power. These limitations of third-rail systems have largely restricted their use to mass transit systems. Capacity is also limited by speed restrictions – 160 km/h (100 mph) is considered to be the maximum speed at which a contact shoe can reliably collect power^{[citation needed]}.

By comparison, overhead wires can provide 25kV or even 50kV, and can take roughly ten times the power^{[citation needed]}.

Infrastructure restrictions

Junctions and other pointwork make it necessary to leave gaps in the live rail at times, as do level crossings. This is not usually a problem, as most third-rail rolling stock has multiple current collection shoes along the length of the train, but under certain circumstances it is possible for a train to become "gapped" - stalled with none of its shoes in contact with the live rail. When this happens, it is usually necessary for the train to be shunted back onto a live section either by a rescue locomotive or another service train, although in some circumstances it is possible to use jumper cables to temporarily hook the train's current collectors to the nearest section of live rail. Especially given that gapping tends to happen at complex, important junctions, it can be a major source of disruption. On the Chicago Transit Authority system, the jumper cables are known as stingers; they are insulated poles with a wired contact that may be manually pressed against contact shoes to restart a gapped train. Other such problems are implementation-specific, usually have workarounds. Another infrastructure restriction of third rail is that the rail and its safety cover decreases the structure gauge and in turn the loading gauge, potentially blocking access to certain types of equipment.

When David Gunn became General Manager of the Washington DC Metro Rail system, he publicly proposed to alleviate crowding by running much more frequent trains as two-car trains instead of the practice the transit authority had of running four-car trains. He had to publicly drop this idea, with some embarassment, when it was pointed out to him that two-car trains can only operate in specific areas of the system, because each car only has one "shoe", on the same side of the car, and even with the practice of having each car pointing in the opposite direction so that there is a shoe on each side of the train, there are many places in the system where a two car train would end up with both "shoes" unable to reach a third rail, stranding the train.

Inefficient contact

Fallen leaves, snow and other debris on the conductor rail can reduce the efficiency of the contact between the conductor rail and the pickup shoes, leaving trains stalled because of the lack of power. However, the bottom-contact third rail, as used on the Metro-North Railroad (see Technical aspects above), and numerous other transit systems including the Docklands Light Railway in London and the Market-Frankford Line in Philadelphia, is highly resistant to this problem.

Older systems adopted top-contact third rail before they realised that there would be problems with leaves, etc., while newer systems have learned from this mistake and use side or bottom contact. However, some relatively new systems in North America, such as the TTC in Toronto, use top-covered top-contact third rails on above-ground portions of its subway system; rarely is the system delayed by electrical problems even after heavy snows. Rather, problems generally arise in other aspects of the system (frozen switches for example) long before snow interferes significantly with electrical pickup. Some systems are less susceptible to this problem due to having mostly underground trackage, or less severe weather.

Compromise systems

Several systems use third rail for part of the system, and other systems such as overhead catenary or diesel power for the remainder. These may exist because of the connection of separately-owned railways using the different systems, local ordinances, or other historical accidents.

USA

In New York City, electric trains that must use third rail leaving Grand Central Terminal on the former New York Central Railroad (now Metro-North Railroad) switch to overhead lines at Pelham when they need to operate out onto the former New York, New Haven and Hartford Railroad (now Metro North's New Haven Line) line to Connecticut. The switch is made "on the fly" controlled from the engineer's position.

Also in New York City, due to a prohibition on diesel emissions in tunnels, Metro-North and Amtrak use diesel locomotives that can also be electrically powered by third-rail. This locomotive, the P32AC-DM, can transition between the two modes while underway. The third-rail auxiliary system is not as powerful as the diesel engine, so on open-air (non-tunnel) trackage run the engines typically run in diesel mode, even where third rail power is available.

In Manhattan, New York City, and in Washington, D.C., local ordinances required electrified street railways to draw current from a third rail and return the current to a fourth rail, both installed in a continuous vault underneath the street and accessed by means of a collector that passed through a slot between the running rails. When streetcars on such systems entered territory where overhead lines were allowed, they stopped over a pit where a man detached the collector (plow) and the motorman placed a trolley pole on the overhead. Some sections of the former London tram system also used the conduit current collection system, also with some tramcars that could collect power from both overhead and under-road sources.

The Blue Line of Boston's MBTA uses third rail electrification from the start of the line downtown to Airport, where it switches to overhead catenary for the remainder of the line to Wonderland. Dual power supply method was also used on some US interurban railways that made use of newer third rail in suburban areas, and existing overhead streetcar (trolley) infrastructure to reach downtown, for example the Skokie Swift in Chicago.

UK

See also: Railway electrification in Great Britain

Several types of British trains operate on both overhead and third rail systems, including class 313, 319, 325 and 373 Eurostar trains.

The Eurostar uses overhead electrical power (at 25 kV AC) in the Channel Tunnel and along the CTRL, with a pantograph height change required between the CTRL line and the Channel Tunnel (which is a unique height). (The trains are also required to cope with height changes for the French urban and TGV lines). In the south-east a transition is made on-the-fly to 750 V DC for the remainder of the journey through the London suburbs, on the standard commuter lines into Waterloo using the third rail system. From 2007, upon completion of the Channel Tunnel Rail Link, there will be overhead electricity all the way into London.

Also in London, the North London Line changes its power supply several times between Richmond and Stratford^{[citation needed]}. The cross-city Thameslink service runs on Southern Region third rail from Farringdon station southwards and on overhead line northwards from Farringdon up to Bedford: this change-over is made while stationary.

Continental Europe

The older lines in the west of the Oslo T-bane system were built with overhead lines (some since converted to third rail) while the eastern lines were built with third rail. Trains operating on the older lines can operate both with third rail and overhead lines. To mitigate investment costs, the Rotterdam Metro, basically a third-rail powered system, has been given some outlying branches built on surface as light rail (called 'Sneltram' in Dutch), with numerous level crossings protected with barriers and traffic lights. These branches have overhead wires. Similarly, in Amsterdam one 'Sneltram' route goes on Metro tracks and passes to surface alignment in the suburbs, which it shares with standard trams. In most recent developments, the RandstadRail project also requires Rotterdam Metro trains to run under wires on their way along the former mainline railway to The Hague.

The newly built tramway in Bordeaux (France) uses a novel system with a third rail in the center of the track. The third rail is separated into 8 m (26 ' 3 ") long conducting and 3 m (9 ' 10 ") long isolation segments. Each conducting segment is attached to an electronic circuit which will make the segment live once it lies fully beneath the tram (activated by a coded signal sent by the train) and switch it off before it becomes exposed again. This system (called "Alimentation par Sol" (APS), meaning "current supply via ground") is used in various locations around the city but especially in the historic centre: elsewhere the trams use the conventional overhead lines, see also ground-level power supply. In summer 2006 it was announced that two new French tram systems would be using APS over part of their networks. These will be Angers and Reims, with both systems expected to open around 2009 / 2010.

Conversions from and to third rail

Despite various technical possibilities of operating stock with dual power collecting modes, the desire to achieve full compatibility of entire networks seems to have been the decisive cause of conversions from third rail to overhead supply (or vice versa).

Selected suburban corridors in Paris, focusing at Gare Saint-Lazare, Gare des Invalides (both CF Ouest) and Gare d'Orsay (CF PO), were electrified from 1924, 1901, 1900 respectively. They all changed to overhead wires by stages after they became part of a wide scale electrification project of the SNCF network (the 1960s-70s).

In Manchester area, the aforementioned Bury Line (originally L&YR) was first electrified with overhead wires (1913), then changed to third rail (1917, cf. Railway electrification in Great Britain) and again in 1992 to overhead wires in the course of its adaptation for the Manchester Metrolink. Trams in city centre streets, carrying collector shoes projecting from their bogies, were considered too dangerous for pedestrians and motor traffic to attempt dual-mode technology (in Amsterdam and Rotterdam Sneltram vehicles go out to surface in suburbs, not in busy central areas). The same thing happened to the West Croydon - Wimbledon Line in Greater London (originally electrified by the Southern Railway) when Croydon Tramlink was built (opened 2000).

Three lines of five making up the core of Barcelona Metro network changed to overhead power supply from third rail. This operation was also done by stages and completed in 2003.

Quite the opposite thing took place in London. The South London Line of the LBSCR network (between Victoria and London Bridge Stations) was electrified with catenary in 1909 - the system was later extended to Crystal Palace, Coulsdon North and Sutton. In the course of mainline third rail electrification in south-east England, the lines were converted accordingly by 1929.

The first overhead electric trains appeared on the de:Hamburg-Altonaer Stadt- und Vorortbahn in 1907. Thirty years later, the mainline railway operator, Deutsche Reichsbahn, influenced by the success of the third-rail Berlin S-Bahn, decided to switch what was now called Hamburg S-Bahn to third rail. The process began in 1940 and was not finished until 1955.

In 1976-1981 the third-rail Viennese U-Bahn U4 Line substituted the Donaukanallinie and Wientallinie of the Stadtbahn, built c1900 and first electrified with overhead wires in 1924. This was part of a big project of consolidated U-Bahn network construction. The other electric Stadtbahn line, whose conversion into heavy rail stock was rejected, still operates under wires with light rail cars (as U6), though it has been thoroughly modernised and significantly extended. As the platforms on the Gürtellinie were not suitable for raising without much intervention into historic Otto Wagner's station architecture, the line would anyway remain incompatible with the rest of the U-Bahn network. Therefore an attempt of conversion to third rail would have been pointless. In Vienna, paradoxically, the wires were retained for aestetic (and economic) reasons.

The already discussed Skokie Swift of Chicago 'L' changed to third rail in 2004, to make it compatible with the rest of the system.

The reasons for building the overhead powered Tyne & Wear Metro network roughly on lines of the long-gone third-rail Tyneside Electrics system in Newcastle area are likely to have roots in economy and psychology rather than in the pursue of compatibility. At the time of the Metro opening (1980) there were no third-rail light rail vehicles on the market and the latter technology was confined to much more costly heavy rail stock. Also the far-going change of image was desired: the memories of the last stage of operation of the Tyneside Electrics were far from being favourable. This was the construction of the system from scratch after eleven years of ineffective diesel service.

Third rails in model trains

The famous Lionel electric trains were the first model railroad sets to use a third rail to power the locomotive starting in 1906. Most model railroad sets only use two rails, where one rail was the "hot" and the other rail was ground. The problem with this setup was that if a loop was designed in the layout where the train would return to the same track in a different direction, a short circuit would be produced where the loop would meet. Even if the loop was isolated, the train would get stuck going back and forth, as the locomotive reversed directions when the polarity reversed underneath it.

The Lionel track used a third rail in the center as the "hot" rail to solve this problem. The train grounded on the two side rails. This design allowed any configuration that could be constructed to be used because the polarity did not reverse with track direction. Converting alternating current to direct current was a complex process in the early days of electricity. Because of this, Lionel trains used alternating current, and still do to this day despite the fact that many newer, smaller 2-rail scales like HO and N operate with direct current like real trains. The use of alternating current means that a Lionel locomotive cannot be reversed by changing the polarity of power. Instead, to reverse the direction, a small transmission was used that would shift the direction each time the locomotive came to a complete stop.

Model trains experience the same voltage-drop issues that full-size trains experience. Obviously, model trains must operate the track at safe enough voltages so that they will not be a shock hazard if contacted, usually around 12 volts or below. This requires a transformer just like in a real railroad system. Just like in a full-size train, when the locomotive is farthest away from the transformer, it receives less voltage, and will slow down. Extremely large model train layouts, just like real electric trains, will frequently have multiple transformers located around the layout to maintain sufficient voltage on the track.

The big difference between most model railroad systems and real trains is the means of speed control. In a real train, its speed is controlled within the train itself. On a model railroad, speed is varied by varying the voltage at the transformer before it is applied to the track. Some of the newest, innovative model railroad sets operate just like real trains and control their speed at the locomotive itself. Just like in a real electric train, a continuous voltage is fed to the third rail, and along with the power is also a data signal that the locomotive(s) receive. The data signal contains an address to tell which locomotive the signal is for, and a control signal to tell the locomotive what speed it is to operate at. A small computer within the model locomotive decodes the signal and adjusts the speed according to the command using voltage regulating transistors and other electronics. This system allows any variety of locomotives to operate on the same track, and at different speeds. They are also realistic in their operation and management in the fact that in a complex model railroad, there exists the risk of collisions between trains operating at different speeds.

List of Systems using third rail

See List of rail transport systems using third rail for a complete list.

See also

Railway electrification
List of systems · Railway electric traction · Traction power network · Electric locomotive · EMUs · Electro-diesel locomotive Overhead lines · Third rail By Country : Great Britain · Japan · USA

List of rail transport systems using third rail Conduit current collection Ground level power supply Rubber-tired metro Third-rail power for trams

List of rapid transit systems List of suburban and commuter rail systems Railway electrification in Great Britain Fourth rail Third rail (model railroading)

External links

• Thomas Edison's third rail patent (1882)
• Lightrail without wires - Paper on Bordeaux' new Tram with street level third rail (by the Transportation Research Board of the National Academies)

• Details of the UK 3rd/4th rail design.
• The "third rail" A short history.