The London Underground uses a 4-rail system where both the conductor rails are live relative to the running rails (the rails used by the train's wheels) though the positive rail has twice the voltage of the negative rail. Arcs like this are quite normal and occur when the electric power collection "shoes" of a train that is motoring (ie: drawing power) reach the end of a section of electric power rail.
The third rail is located either in between the two running rails or, usually, on the outside of 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 (side running) or bottom (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.
The third rail is an alternative to electrified overhead lines that transmit power to trains by means of pantograph arms attached to the trains. On some metro/light rail lines as well as regional rail lines, part of the line has a third rail and another part overhead wires, and vehicles allow both, e.g. in Rotterdam, Boston's Blue Line, North London Line, Milan subway (line M1) or Metro-North's New Haven Division (Commuter rail in North America). 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 (suburban trains in Hamburg), and direct current (DC) is used.
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 Montréal 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. A third rail (current feed, outside the running rails) and fourth rail (current return, half way between the running rails) design, that has other advantages, is used by a few steel-wheel systems. The London Underground is the largest of these, see Fourth Rail.
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.
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 two marketed ways of attaching the stainless steel to the aluminium. The oldest being a co-extruded method where the stainless steel is extruded with the aluminum. This co-extruded method has suffered, in isolated cases, from de-lamination (where the stainless steel seperates from the aluminium) this is said to have been eliminated in the latest co-extruded rails. The second method is a 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 positvely locked to provide a good current collection interface. The photo on the right depicts such a rail.
Advantages of third rail
Cost
Third-rail systems are less expensive 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.
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 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.
Convention
While sometimes used in new transit system construction, third rails are now considerably less popular than overhead systems. In the U.S., they are still the usual means of powering heavy rapid transit lines that are completely grade-separated. Monorail typically use a variation of the third rail (with a fourth rail as well) for current transmission in the form of cables or other electrical conductors placed on the sides of the guideway and contacted by a sprung shoe. Many older railways still use third rails and DC power, even where overhead lines would otherwise be practical, due to the high cost of retrofitting.
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. 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 continous stream has been proven to be unable to conduct electricity by MythBusters [2].
- 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 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.
- As the above example shows, this factor is highly dependent on the specific transit system's implementation. As another example, the BART system in the San Francisco Bay Area uses sturdy sheaths to cover its third rails and always places the rail on the further side of the track away from where passengers would normally be. As a result, some stations have the third rail on the left relative to the train's motion, while others have it on the right. If someone falls on the tracks, this allows them to safely return to the platform without the danger of stepping on the third rail or crawl under the platform into a special "Safety Area" if a train is too close to safely climb back up. The New York City Subway, Washington Metro and Long Island Rail Road follow similar procedures.
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 this low voltage means that electrical feeder sub-stations have to be set up at frequent intervals along the line, 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. Even higher voltages, such as 750V DC third rail is used on many hundreds of suburban railway route miles across south and southeast England, and the 1000V DC used on the BART system, with just over 100 miles of track, are restricted to this area of railroad transport. 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.
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. Again, such problems are very much implementation-specific, and transit systems around the world have managed to work around these problems. The aforementioned BART system has numerous sections of switching track, especially around its transfer stations, where the third rail is alternately on the right- and left-hand sides. No problems have arisen from the use of this system in years. On the Metro-North Railroad and the Long Island Rail Road (including Pennsylvania Station which is owned by Amtrak), the safety cover decreases the structure gauge and in turn the loading gauge.
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), BART, and numerous other transit systems including the Docklands Light Railway in London, is highly resistant to this problem. For example, the BART system contains almost 70 miles of above-ground track and experiences few, if any, problems as a result of weather. In addition, systems that are completely covered (i.e. underground) are obviously immune to the problem. Basically, 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. It should be pointed out, however, that some systems, such as the TTC in Toronto, use 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 from other aspects of the system (frozen switches for example) long before snow interferes significantly with electrical pickup.
Compromise systems
There are and have been several systems in which third rail has been used for part of the system, and overhead lines for the remainder. These exist sometimes because of the connection of separately-owned railways using the different systems, or because of local ordinances.
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.
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.
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.
Several types of British Railway trains operate on both overhead and third rail systems, including the class 313, 319, 325 and 373 Eurostar trains.
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, and here too there were some tramcars which could collect power from both overhead and under-road sources.
The newly built Tramway in the City of Bordeaux (France) uses a novel system with a third rail in the center of the track. The third rail is separated into 8m (26ft-3in) long conducting and 3m (9ft-10in) 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 suburban locations around the city as well as 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.
In Chicago, the Yellow Line, also known as the Skokie Swift, operated for most of its distance with third rail, switching to overhead catenary before reaching the end of the line at the Dempster Street station. In 2004, the catenary portion was converted to third rail. This particular line was once a part of the Chicago, North Shore and Milwaukee interurban line.
In south east England the Eurostar switches from overhead lines when leaving the Channel Tunnel and continues its journey to London on the standard commuter lines using the thrid rail system. However this problem will be overcome in 2007 with the completion of the Channel Tunnel Rail Link which will use overhead lines all the way.
Also in London, the North London Line changes its power supply several times on the way from Richmond to North Woolwich. And the Thameslink service runs on Southern Region third rail from Farringdon station southwards and on overhead lines northwards from Farringdon to Bedford.
Systems using third rail
Argentina
Austria
Belgium
Brazil
Egypt
Finland
France
Germany
Norway
Portugal
Romania
Singapore
Sweden
The Netherlands
Ukraine
United Kingdom
United States
See also
External links
- Details of the UK 3rd/4th rail design.